Sheet glass forming apparatus

ABSTRACT

The present invention alters the flow path at the inlet of the sheet glass forming apparatus to improve quality. The bottom of the downcomer pipe is preferably shaped to alter the character of the vortex flow in the quiescent flow zone between the pipes. In another embodiment, a bead guide provides hydraulic stresses that are in opposition to the surface tension stress and thus reduces the influence of surface tension on the formation of thick beads on the edges of the sheet. The present invention also measures the temperature of the glass by immersing thermocouples in the glass, at locations where any defects caused by the immersion are in the glass that forms the unusable edges of the sheet. In another embodiment, the support structure for the trough is altered to substantially reduce the aging of the trough due to thermal creep.

REFERENCE TO RELATED APPLICATIONS

This application claims an invention that was disclosed in one of thefollowing provisional applications:

-   -   1) Provisional Application No. 60/444,728, filed Feb. 4, 2003,        entitled “SHEET GLASS FORMING APPARATUS”;    -   2) Provisional Application No. 60/449,671, filed Feb. 24, 2003,        entitled “SHEET GLASS FORMING APPARATUS”;    -   3) Provisional Application No. 60/505,302, filed Sep. 23, 2003,        entitled “TEMPERATURE MEASUREMENT FOR SHEET GLASS FORMING        APPARATUS”; and    -   4) Provisional Application No. 60/534,950, filed Jan. 8, 2004,        entitled “SHEET GLASS FORMING APPARATUS”.

The benefit under 35 USC § 119(e) of the United States provisionalapplications is hereby claimed, and the aforementioned applications arehereby incorporated herein by reference.

In addition, this application is a continuation in part of copendingU.S. application Ser. No. 10/214,904, filed on Aug. 8, 2002, entitled“SHEET GLASS FORMING APPARATUS”. The aforementioned application ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains generally to the manufacture of glass sheets,and, more particularly, to glass sheets formed from an overflow process.

2. Description of Related Art

The glass that is used for semiconductor powered display applications,and particularly for TFT/LCD display devices that are widely used forcomputer displays, must have very high surface quality to allow thesuccessful application of semiconductor type material. Sheet glass madeusing the apparatus of U.S. Pat. No. 3,338,696, assigned to Corning,Inc., makes the highest quality glass as formed and does not requirepost-processing. The Corning patent makes glass by a manufacturingprocess termed: “The Overflow Process”. Glass made using other processesrequires grinding and/or polishing and thus does not have as fine asurface finish. The glass sheet must also conform to stringent thicknessvariation and warp specifications. The fine surface finish is formedfrom virgin glass primarily from the center of the glass stream. Thisglass has not been in contact with foreign surfaces since the stirringoperation.

The teachings of U.S. Pat. No. 3,338,696 are still the state of the artas practiced today. However, the apparatus has limitations.

A major drawback of the apparatus of “The Overflow Process” is that,even though it makes excellent glass over most of the surface, thesurface of the glass sheet nearest the inlet is composed of glass thathas flowed in proximity to the feeding pipe surfaces and therefore issubject to lower quality.

Another drawback of the apparatus of “The Overflow Process” is that,even though its makes excellent glass during stable operatingconditions, it recovers from transient conditions very slowly. This iscaused in part by quiescent zones of glass flow in the pipes conductingthe glass from the stirring device to the apparatus when these pipes aredesigned using traditional practice. During unintended process transientthese quiescent zones slowly bleed glass of a previous materialcomposition into the main process stream of glass causing defects. Thesedefects eventually subside when the process stabilizes; however, thereis a period of time where the quality of the glass sheet is substandard.

Yet another drawback of the apparatus of “The Overflow Process” is thelimited means for controlling the thickness of the formed sheet. Theselective cooling of the glass with respect to width as the sheet isformed is not provided in current practice.

The thickness control system of U.S. Pat. No. 3,682,609 can compensatefor small thickness errors, but it can only redistribute the glass overdistances on the order of 5-10 cm.

Another drawback of the apparatus of “The Overflow Process” is that theforming apparatus deforms during a manufacturing campaign in a mannersuch that the glass sheet no longer meets the thickness specification.This is a primary cause for premature termination of the production run.

A further drawback of the apparatus of “The Overflow Process” is thatsurface tension and body forces have a major effect on the molten glassflow down the external sides of the forming apparatus causing the sheetto be narrower than the forming apparatus and the edges of the formedsheet to have thick beads.

U.S. Pat. No. 3,451,798 provides for edge directors which endeavor tocompensate for the surface tension effects but are in reality acorrection for problems created by restricting the forming apparatuscross-section to a single profile on its external surface. These edgedirectors often extend below the bottom of the trough providing someinfluence to flow below the trough; however, the fixed shape, lack ofadjustability and lack of a heating option limit their usefulness inminimizing and/or eliminating the bead at each edge of the sheet.

Another drawback of the prior art is that the glass sheet is notinherently flat when drawn from the forming apparatus.

Yet another drawback of the apparatus of “The Overflow Process” is thatthe drawing of the sheet from the bottom of the apparatus has apropensity to have a cyclic variation in sheet thickness. This cyclicthickness variation is a strong function of uncontrolled air currents,which tend to become more prevalent as the equipment ages during aproduction campaign. As the apparatus ages, air leaks develop throughcracks in material and assorted openings caused by differentialexpansion.

An additional drawback of the apparatus of “The Overflow Process” isthat, even though it makes excellent glass during stable operatingconditions, the control of process temperature and flow is limited bypoor measurement technology, thus allowing unintended process transientswhich cause glass defects. Temperature measurement technology aspresently practiced measures the temperature of the outer surface of theprocess piping, not the actual temperature of the entering glass processstream. Defects caused by the flow and temperature transients eventuallysubside when the process stabilizes; however, there is a period of timewhere the quality of the glass sheet is substandard.

Another drawback of the apparatus of “The Overflow Process” is that thetemperature of the glass in the forming apparatus is not measured. Onlythe temperature of the air in the chamber surrounding the apparatus ismeasured.

Therefore, there is a need in the art for an apparatus which overcomesthe shortcomings of the prior art.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, all the glass thatforms the surface of the useful area of the sheet is virgin glass, whichis not contaminated by flow in proximity to a refractory or refractorymetal surface after the stirring operation. In addition, this embodimentsignificantly reduce inhomogeneities in the glass that forms the sheetby relocating or eliminating the regions of quiescent flow in the pipingbetween the stirring device and the sheet glass forming apparatus.

The present invention alters the flow path at the inlet of the sheetglass forming apparatus to improve quality. The vortex flow is alteredat the inlet to the apparatus. The bottom of the downcomer pipe ispreferably shaped to alter the character of the vortex flow in thequiescent flow zone between the pipes. The shape of the bottom end ofthe downcomer pipe is not flat or linear. The altered shape has at leastone downward extension, which is curved. In a preferred embodiment,there are three downward extensions. The downward extensions arepreferably V-shaped.

In a preferred embodiment, the present invention significantly reducesinhomogeneities in the glass that forms the useable sheet by divertingthe glass from the quiescent flow region at the joint of the downcomerpipe and the inlet pipe to the unusable edges of the sheet.

In another preferred embodiment, this invention introduces a precisethermal control system to redistribute the flow of molten glass at theweirs which is the most critical area of the forming process. Thisthermal control effectively counteracts the degradation of the sheetforming apparatus which inevitably occurs during a production campaign.

In yet another preferred embodiment, the invention introduces acounteracting force to these stresses on the trough in a manner suchthat the thermal creep which inevitably occurs has a minimum impact onthe glass flow characteristics of the forming trough. This embodiment isdesigned such that this counteracting force is maintained through anextended period of the production campaign. Thus, sheet glass may bemanufactured for a longer time with the same forming trough.

Another preferred embodiment creates a variable external cross-sectionwhich alters the direction and magnitude of the surface tension and bodyforce stresses and thus, reduces the adverse influence of surfacetension and body forces on sheet width.

An embodiment of the present invention embodies design features thatlimit the effect of surface tension on the flow off the bottom of theforming apparatus. It also provides additional versatility in theadjustment of the thickness of the glass beads at each end of the sheet.

In a preferred embodiment, the present invention employs a bead guide.The bead guide is an adjustably shaped device located beneath the troughat each end of the trough. This device is easily removable forreplacement or modification during a production run. The molten glassflows over and attaches itself to the bead guide. The device isoptionally heated. The bead guide provides hydraulic stresses that arein opposition to the surface tension stress and thus reduces theinfluence of surface tension on the formation of thick beads on theedges of the sheet.

In an alternative embodiment, the bead guide is used in combination withan altered trough design. The width and the angle of the inverted slopeof the forming wedge are changed to alter the effect of surface tensionand body forces on the narrowing of the sheet. In addition, the widthand the inverted slope angle may be increased to make the structurestiffer and thus more resistant to thermal creep. The variable externalcross-section provides hydraulic stresses that are in opposition to thesurface tension and body force stresses and thus reduces the influenceof surface tension and body forces.

In an alternative preferred embodiment, the glass is preferentiallycooled across its width to create forming stresses duringsolidification, which ensure that the glass sheet drawn is inherentlyflat.

In a further preferred embodiment, this invention adjusts the internalpressure in each of the major components of the forming apparatus suchthat the pressure difference across any leakage path to the forming zoneis essentially zero. Therefore, air leakage in the apparatus isminimized even though the cracks and openings exist during initialoperation and develop during manufacturing.

A preferred embodiment of the present invention measures the temperatureof the glass by immersing thermocouples in the glass, at locations fromwhich any defects caused by the immersion are in the glass that formsthe unusable edges of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principle parts of “The Overflow Process” glasssheet manufacturing system.

FIG. 2A shows a side view of “The Overflow Process” as known in theprior art.

FIG. 2B shows a cross-section of the glass flow in the downcomer pipeacross lines B-B of FIG. 2A.

FIG. 2C shows a cross-section across lines C-C of FIG. 2A, where theglass flow in the downcomer pipe appears in the sheet for “The OverflowProcess”.

FIG. 3A shows a side view of a surface flow distribution device in apreferred embodiment of the present invention.

FIG. 3B shows a top view of a surface flow distribution device in apreferred embodiment of the present invention.

FIG. 4A shows a side view of a submerged flow distribution device in apreferred embodiment of the present invention.

FIG. 4B shows a top view of a submerged flow distribution device in apreferred embodiment of the present invention.

FIG. 5A shows a side view of “The Overflow Process” in an embodiment ofthe present invention.

FIG. 5B shows the glass flow in the downcomer pipe across lines B-B ofFIG. 5A when a flow distribution device is used.

FIG. 5C shows a cross-section across lines C-C of FIG. 5A, where theglass flow in the downcomer pipe appears in the sheet when a flowdistribution device is used.

FIG. 6 shows a bowl with an inclined axis which diffuses the quiescentflow zone at the bowl nose in a preferred embodiment of the presentinvention.

FIG. 7A shows the top view of a bowl with side inflow which relocatesthe quiescent flow zone from the bowl nose to the bowl side in apreferred embodiment of the present invention.

FIG. 7B shows a side view of FIG. 7A.

FIG. 7C shows the top view of a bowl with side inflow which relocatesthe quiescent flow zone from the bowl nose to a location approximately45 degrees to the side with respect to the centerline of the formingapparatus in a preferred embodiment of the present invention.

FIG. 7D shows a side view of FIG. 7C.

FIG. 8 illustrates a bowl in “The Overflow Process” as known in theprior art.

FIG. 9A shows a downcomer pipe feeding the forming apparatus inlet withminimum quiescent flow in a preferred embodiment of the presentinvention.

FIG. 9B shows a top view of FIG. 9A.

FIG. 9C shows a detail of the downcomer pipe to trough inlet pipeconnection showing the glass flow pattern in a preferred embodiment ofthe present invention.

FIG. 10A shows the flow between the downcomer pipe and the formingapparatus inlet in “The Overflow Process” as known in the prior art.

FIG. 10B shows a top view of FIG. 10A.

FIG. 10C shows a detail of the downcomer pipe to trough inlet pipeconnection showing the glass flow pattern as shown in the prior art.

FIG. 11A shows the principle parts of a typical “Overflow Process”manufacturing system.

FIG. 11B shows a section of FIG. 11A.

FIG. 12A illustrates a side view of the glass flowing through theforming trough.

FIG. 12B shows a section through the center of the forming trough ofFIG. 12A showing the cooling zones.

FIG. 13A shows a revised single heating chamber muffle design in apreferred embodiment of the present invention.

FIG. 13B shows a section of FIG. 13A.

FIG. 14A shows air cooling tubes to effect localized cooling to themolten glass as it passes over the weirs in a preferred embodiment ofthe invention.

FIG. 14B shows a section of FIG. 14A.

FIG. 15A shows a muffle with multiple heating chambers in a preferredembodiment of the invention.

FIG. 15B shows a section of FIG. 15A.

FIG. 16A shows radiant coolers which effect localized cooling to themolten glass as it passes over the weirs in a preferred embodiment ofthe invention.

FIG. 16B shows a section of FIG. 16A.

FIG. 17A illustrates how the prior art trough design deforms as a resultof thermal creep.

FIG. 17B shows another view of FIG. 17A.

FIG. 18A shows the forming trough support system as known in the priorart.

FIG. 18B shows another view of FIG. 18A.

FIG. 18C shows another view of FIG. 18A.

FIG. 18D shows another view of FIG. 18A.

FIG. 19A shows single shaped compression blocks on each end of thetrough in a preferred embodiment of the present invention.

FIG. 19B shows another view of FIG. 19A.

FIG. 19C shows another view of FIG. 19A.

FIG. 19D shows another view of FIG. 19A.

FIG. 20A show a single shaped compression block on one end of the troughand multiple shaped compression blocks on the other end in a preferredembodiment of the present invention.

FIG. 20B shows another view of FIG. 20A.

FIG. 20C shows another view of FIG. 20A.

FIG. 20D shows another view of FIG. 20A.

FIG. 21A shows a trough design as known in the prior art.

FIG. 21B shows a top view of FIG. 21A.

FIG. 21C shows a cross-section of the trough design shown in FIG. 21Aacross lines C-C.

FIG. 21D shows a cross-section of the trough design shown in FIG. 21Aacross lines D-D.

FIG. 21E shows a cross-section of the trough design shown in FIG. 21Aacross lines E-E.

FIG. 21F shows a cross-section of the trough design shown in FIG. 21Aacross lines F-F.

FIG. 21G shows a cross-section of the trough design shown in FIG. 21Aacross lines G-G.

FIG. 22A shows a reduced inverted slope at each end of the trough in apreferred embodiment of the present invention.

FIG. 22B shows a top view of FIG. 22A.

FIG. 22C shows a cross-section of the trough design shown in FIG. 22Aacross lines C-C.

FIG. 22D shows a cross-section of the trough design shown in FIG. 22Aacross lines D-D.

FIG. 22E shows a cross-section of the trough design shown in FIG. 22Aacross lines E-E.

FIG. 22F shows a cross-section of the trough design shown in FIG. 22Aacross lines F-F.

FIG. 22G shows a cross-section of the trough design shown in FIG. 22Aacross lines G-G.

FIG. 23A shows an alternate embodiment of the present invention withfurther modified ends.

FIG. 23B shows a top view of FIG. 23A.

FIG. 23C shows a cross-section of the trough design shown in FIG. 23Aacross lines C-C.

FIG. 23D shows a cross-section of the trough design shown in FIG. 23Aacross lines D-D.

FIG. 23E shows a cross-section of the trough design shown in FIG. 23Aacross lines E-E.

FIG. 23F shows a cross-section of the trough design shown in FIG. 23Aacross lines F-F.

FIG. 23G shows a cross-section of the trough design shown in FIG. 23Aacross lines G-G.

FIG. 24A shows an alternate embodiment of the present invention with thepotential for increased structural stiffness.

FIG. 24B shows a top view of FIG. 24A.

FIG. 24C shows a cross-section of the trough design shown in FIG. 24Aacross lines C-C.

FIG. 24D shows a cross-section of the trough design shown in FIG. 24Aacross lines D-D.

FIG. 24E shows a cross-section of the trough design shown in FIG. 24Aacross lines E-E.

FIG. 24F shows a cross-section of the trough design shown in FIG. 24Aacross lines F-F.

FIG. 24G shows a cross-section of the trough design shown in FIG. 24Aacross lines G-G.

FIG. 25A shows a forming trough with a convex upward forming root whichsolidifies the center glass before the edge glass in a preferredembodiment of the present invention.

FIG. 25B shows another view of FIG. 25A.

FIG. 25C shows another view of FIG. 25A.

FIG. 25D shows another view of FIG. 25A.

FIG. 26A shows a forming trough with a convex downward forming rootwhich solidifies the edge glass before the center glass in a preferredembodiment of the present invention.

FIG. 26B shows another view of FIG. 26A.

FIG. 26C shows another view of FIG. 26A.

FIG. 26D shows another view of FIG. 26A.

FIG. 27A shows a forming trough with a complexly shaped forming root tosolidify the glass in a unique manner over its width in a preferredembodiment of the present invention.

FIG. 27B shows another view of FIG. 27A.

FIG. 27C shows another view of FIG. 27A.

FIG. 27D shows another view of FIG. 27A.

FIG. 28A illustrates the cooling process in “The Overflow Process” glasssheet forming system as known in the prior art.

FIG. 28B shows a section of FIG. 28A.

FIG. 29A shows how the pressure in the muffle zone may be controlled tominimize leakage in a preferred embodiment of the present invention.

FIG. 29B shows a section of FIG. 29A.

FIG. 30A shows how the pressure in the muffle door zone may becontrolled to minimize leakage in a preferred embodiment of the presentinvention.

FIG. 30B shows a section of FIG. 30A.

FIG. 31A shows how the pressure in the transition zone may be controlledto minimize leakage in a preferred embodiment of the present invention.

FIG. 31B shows a section of FIG. 31A.

FIG. 32A shows how the pressure in the annealer and pulling machine zonemay be controlled to minimize leakage in a preferred embodiment of thepresent invention.

FIG. 32B shows a section of FIG. 32A.

FIG. 33A shows a downcomer pipe and inlet pipe as known in the priorart.

FIG. 33B shows streamlines of glass flow as known in the prior art.

FIG. 34A shows a downcomer pipe and an inlet pipe which are notconcentric in an embodiment of the present invention.

FIG. 34B shows streamlines of glass flow when the downcomer and inletpipes are not concentric in the embodiment of FIG. 34A.

FIG. 35A shows a downcomer pipe with a single downward extensionsubmerged in the inlet pipe glass in an embodiment of the presentinvention.

FIG. 35B shows a downcomer pipe with a single downward extensionpartially submerged in the inlet pipe glass in an embodiment of thepresent invention.

FIG. 35C shows a downcomer pipe with a single downward extensionsubstantially above the glass free surface in the inlet pipe glass in anembodiment of the present invention.

FIG. 36A shows a downcomer pipe with a single downward extensionsubmerged in the inlet pipe glass in an embodiment of the presentinvention.

FIG. 36B shows the streamlines of glass flow in the embodiment shown inFIG. 36A.

FIG. 37A shows a downcomer pipe with a single downward extension offcenter and submerged in the inlet pipe glass in an embodiment of thepresent invention.

FIG. 37B shows the streamlines of glass flow for the embodiment of thepresent invention shown in FIG. 37A.

FIG. 38A shows a downcomer pipe with three downward extensions submergedin the inlet pipe glass in an embodiment of the present invention.

FIG. 38B shows a downcomer pipe with three downward extensions partiallysubmerged in the inlet pipe glass in an embodiment of the presentinvention.

FIG. 38C shows a downcomer pipe with three downward extensionssubstantially above the glass free surface in the inlet pipe glass in anembodiment of the present invention.

FIG. 39A shows a downcomer pipe with three downward extensions submergedin the inlet pipe glass in an embodiment of the present invention.

FIG. 39B shows the streamlines of glass flow in the embodiment of thepresent invention shown in FIG. 39A.

FIG. 40A shows another embodiment of the present invention whereby theshape of the inlet pipe is modified.

FIG. 40B shows a view looking down on the inlet pipe of FIG. 40A.

FIG. 41A shows another view of the inlet pipe of FIG. 40A.

FIG. 41B shows the streamlines of glass flow in the embodiment shown inFIGS. 40A, 40B and 41A.

FIG. 42A shows a downcomer pipe with three downward extensions submergedin the glass of a modified inlet pipe in an embodiment of the presentinvention.

FIG. 42B shows the streamlines of glass flow in the embodiment of thepresent invention shown in FIG. 42A.

FIG. 43A shows a prior art trough design showing the contraction of theglass sheet width.

FIG. 43B shows a cross-section of the trough shown in FIG. 43A acrosslines B-B.

FIG. 43C shows a cross-section of the trough shown in FIG. 43A acrosslines C-C.

FIG. 43D shows a cross-section of the trough shown in FIG. 43A acrosslines D-D.

FIG. 43E shows a cross-section of the trough shown in FIG. 43A acrosslines E-E.

FIG. 43F shows a cross-section of the trough shown in FIG. 43A acrosslines F-F.

FIG. 43G shows a cross-section of the trough shown in FIG. 43A acrosslines G-G.

FIG. 44A shows the effect of a bead guide on the sheet width using thetrough design shown in FIG. 43 in an embodiment of the presentinvention.

FIG. 44B shows a cross-section of the trough shown in FIG. 44A acrosslines B-B.

FIG. 44C shows a cross-section of the trough shown in FIG. 44A acrosslines C-C.

FIG. 44D shows a cross-section of the trough shown in FIG. 44A acrosslines D-D.

FIG. 44E shows a cross-section of the trough design shown in FIG. 44Aacross lines E-E.

FIG. 44F shows a cross-section of the trough shown in FIG. 44A acrosslines F-F.

FIG. 44G shows a cross-section of the trough shown in FIG. 44A acrosslines G-G.

FIG. 45A shows an alternative embodiment of the present invention, witha bead guide located below an improved trough design.

FIG. 45B shows a cross-section of the trough shown in FIG. 45A acrosslines B-B.

FIG. 45C shows a cross-section of the trough shown in FIG. 45A acrosslines C-C.

FIG. 45D shows a cross-section of the trough shown in FIG. 45A acrosslines D-D.

FIG. 45E shows a cross-section of the trough shown in FIG. 45A acrosslines E-E.

FIG. 45F shows a cross-section of the trough shown in FIG. 45A acrosslines F-F.

FIG. 45G shows a cross-section of the trough shown in FIG. 45A acrosslines G-G.

FIG. 46A shows an embodiment of the bead guide of the present invention.

FIG. 46B shows an alternative embodiment of the bead guide of thepresent invention.

FIG. 46C shows an alternative embodiment of the bead guide of thepresent invention.

FIG. 46D shows an alternative embodiment of the bead guide of thepresent invention.

FIG. 47A shows mounting for a rotating bead guide in an embodiment ofthe present invention.

FIG. 47B shows mounting for a rotating bead guide in an alternativeembodiment of the present invention.

FIG. 48A shows preferred locations of immersed thermocouples in thedowncomer pipe and the forming apparatus inlet pipe in an embodiment ofthe present invention.

FIG. 48B is a cross-section through the bowl across line B-B of FIG. 48Afurther detailing the preferred thermocouple locations.

FIG. 48C is a cross-section through the downcomer pipe across line C-Cof FIG. 48A further detailing the preferred thermocouple locations.

FIG. 48D is a cross-section through the trough inlet pipe across lineD-D of FIG. 48A further detailing the preferred thermocouple locations.

FIG. 48E is a cross-section through the bowl inlet pipe across line E-Eof FIG. 48A further detailing the preferred thermocouple locations.

FIG. 49A is a side view of the trough showing thermocouples located onthe bottom of the trough in an embodiment of the present invention.

FIG. 49B is a top view of the trough showing thermocouples located onthe bottom centerline of the trough.

FIG. 50A is a side view of the trough showing thermocouples located inan instrumentation assembly in an embodiment of the present invention.

FIG. 50B is a top view of the trough showing thermocouples located in aninstrumentation assembly on the bottom centerline of the trough.

FIG. 50C shows a side view of the instrumentation assembly.

FIG. 50D shows a top view of the instrumentation assembly.

FIG. 51A is an illustration of the thermal creep deformation of theglass forming trough with no applied load.

FIG. 51B is an illustration of the thermal creep deformation of theglass forming trough under an applied load that corrects for thedeformation.

FIG. 51C is an illustration of the thermal creep deformation of theglass forming trough under too large an applied load.

FIG. 52A illustrates the prior art glass forming trough support system.

FIG. 52B shows a sectional view of FIG. 52A.

FIG. 52C shows a partial view of FIG. 52A.

FIG. 52D shows a sectional view of FIG. 52A.

FIG. 53A illustrates how the prior art glass forming trough deforms as aresult of thermal creep as it is stressed by the prior art troughsupport system.

FIG. 53B shows a sectional view of FIG. 53A.

FIG. 53C shows a partial view of FIG. 53A.

FIG. 53D shows a sectional view of FIG. 53A.

FIG. 54A shows an embodiment of present invention trough support systeminvolving support blocks for the weight of the trough at each end andfree-floating compression blocks at each end.

FIG. 54B shows a sectional view of FIG. 54A.

FIG. 54C shows a partial view of FIG. 54A.

FIG. 54D shows a sectional view of FIG. 54A.

FIG. 55A shows a trough support system involving a support block for theweight of the trough and a floating compression block at the inlet endof the trough and a floating trough weight and trough compression blockat the far end of the trough in an embodiment of the present invention.

FIG. 55B shows a sectional view of FIG. 55A.

FIG. 55C shows a partial view of FIG. 55A.

FIG. 55D shows a sectional view of FIG. 55A.

FIG. 56A shows a force motor added to the top end of the far end of thetrough in an embodiment of the present invention.

FIG. 56B shows a sectional view of FIG. 56A.

FIG. 56C shows a partial view of FIG. 56A.

FIG. 56D shows a sectional view of FIG. 56A.

FIG. 57A shows a retained inlet end adjusting screw and a force motor togenerate a constant sealing force for the glass seal between the inletpipe and the trough in an embodiment of the present invention.

FIG. 57B shows a sectional view of FIG. 57A.

FIG. 57C shows a partial view of FIG. 57A.

FIG. 57D shows a sectional view of FIG. 57A.

FIG. 58A shows the inlet end and outlet end forces being equal in anembodiment of the present invention.

FIG. 58B shows a sectional view of FIG. 58A.

FIG. 58C shows a partial view of FIG. 58A.

FIG. 58D shows a sectional view of FIG. 58A.

DETAILED DESCRIPTION OF THE INVENTION

The flow dynamics of this invention are such that the outside surfacesof the glass sheet are formed from thoroughly mixed virgin glass thatcomes from the center of the glass stream flowing into the formingapparatus and thus has not contacted a refractory or refractory metalsurface. This produces the highest possible surface quality. Thispristine surface is essential for the manufacture of LCD/TFTsemiconductor display devices. In addition, the flow dynamics in allembodiments of this invention are such that the flow rate of moltenglass to the forming wedge at the bottom of the forming trough issubstantially uniform over its width.

Referring to FIGS. 1, 11A and 11B, a typical “Overflow Process”manufacturing system (1) is shown. The glass (10) from the meltingfurnace (2) and forehearth (3), which must be of substantially uniformtemperature and chemical composition, feeds a stirring device (4). Thestirring device (4) thoroughly homogenizes the glass. The glass (10) isthen conducted through a bowl inlet pipe (5), into a bowl (6), and downinto the downcomer pipe (7), through the joint (14) between thedowncomer pipe (7) and the forming apparatus inlet pipe (8), to theinlet of the overflow trough (9). While flowing from the stirring device(4) to the trough (9), the glass (10), especially that which forms thesheet surface, must remain homogeneous. The normal purpose of the bowl(6) is to alter the flow direction from horizontal to vertical and toprovide a means for stopping the flow of glass (10). A needle (13) isprovided to stop glass flow. The normal function of the joint (14)between the downcomer pipe (7) and the trough inlet pipe (8) is to allowfor removal of the sheet glass forming apparatus for service as well asa means of compensation for the thermal expansion of the processequipment.

The molten glass (10) from the melting furnace and forehearth, whichmust be of substantially uniform temperature and chemical composition,enters the forming apparatus through the inlet pipe (8) to the sheetforming trough (9). The inlet pipe (8) is preferably shaped to controlthe velocity distribution of the incoming molten glass flow. The glasssheet forming apparatus, which is described in detail in both U.S. Pat.No. 3,338,696 and application Ser. No. 09/851,627 (filed May 9, 2001,U.S. Patent Publication No. US2001/0039814) and Ser. No. 10/214,904(filed Aug. 8, 2002, U.S. Patent Publication No. US2003/0029199), hereinincorporated by reference, is a wedge shaped forming device (9).Straight sloped weirs (115), substantially parallel with the pointededge of the wedge (root) (116), form each side of the trough. The bottomof the trough (117) and sides of the trough (118) are contoured in amanner to provide even distribution of glass to the top of each sideweir (115). The glass then flows over the top of each side weir (115),down each side of the wedge shaped forming device (9), and joins at thepointed edge of the root (116), to form a sheet of molten glass (11).The sheet of molten glass (11) is then cooled as it is pulled off theroot (116) by pulling rollers (111) to form a solid glass sheet (12) ofsubstantially uniform thickness. Edge rollers (110) may also be used todraw the molten glass sheet (11).

Referring also to FIGS. 43A through 43G, as the glass flows down thevertical portion (436) of the forming wedge, the surface tension andbody forces have a minimal effect on the sheet width, whereas, when themolten glass (10) flows vertically down the inverted slope portion (437)of the forming wedge, the surface tension and body forces act to makethe sheet narrower. This is shown in FIG. 43A as the glass flows on theside of the trough (9) from point (432) to point (433).

In the prior art, the forming trough (9) is encased within a muffle(119) whose purpose is to control the temperature of the forming trough(9) and the molten glass (10). It is normal practice to maintain auniform temperature in the muffle chamber (113) surrounding the formingtrough (9). Cooling the glass as it transitions from the molten state tothe solid state must be carefully controlled. This cooling processstarts on the lower part of the forming apparatus (9) just above theroot (116), and continues as the molten glass sheet passes through themuffle door zone (114). The molten glass is substantially solidified bythe time it reaches the pulling rollers (111). The molten glass forms asolid glass sheet (12) of substantially uniform thickness.

Altering Glass Flow Distribution

Referring also to FIGS. 2 through 10, a preferred embodiment of thepresent invention alters the flow path at the inlet of the sheet glassforming apparatus to improve surface quality. It also facilitates moreuniform flow of glass through the piping that conducts the glass fromthe stirring device to the sheet glass forming apparatus.

U.S. Pat. No. 3,338,696 considers only the glass flow within the formingtrough. U.S. Pat. No. 3,338,696 also claims that the entire sheetsurface is formed from virgin glass, which has not been adverselyeffected by contact with a foreign surface. This is not entirelycorrect, as the sheet formed on the inlet end of the trough has flowedon the piping system front surface. A flow distribution device is addedat the trough inlet in this invention to ensure that all of the useablesheet surface is formed from virgin glass. The piping system between theglass stirring device and the glass sheet forming apparatus is modifiedfrom traditional practice in the bowl and at the collection between thedowncomer pipe and the forming apparatus inlet pipe. The flow throughthe bowl is altered, either eliminating or relocating the quiescent flowzone that normally forms at the front top surface of the bowl. Thedowncomer pipe is not submerged in the forming apparatus inlet pipeglass thus minimizing the quiescent flow zone between the pipes. Thecenterline of the downcomer pipe is moved off center relative to thecenterline of the inlet pipe to alter the vortex flow pattern at thejoint of the two pipes. The shape of the bottom of the downcomer pipeand of the top section of the inlet pipe are also preferably modified toalter the vortex flow pattern at the joint of the two pipes.

FIGS. 2A through 2C illustrate where the glass (10) flowing in thedowncomer feed pipe (7) ends up in the formed glass sheet in the priorart “Overflow Process”. The glass flow in proximity to the sides (21) ofthe downcomer pipe (7) ends up in the center of the drawn sheet. Theflow (23) in proximity to the front surface of the downcomer pipe (7) isdistributed over the entire glass surface, however, it is mostconcentrated on the approximate one third of the sheet at the inlet end.This surface glass (23) is subject to disruption by the downcomer pipesurface and by the glass in the quiescent zones in the bowl (6) and atthe downcomer pipe (7) to inlet pipe (8) collection (14). The surface ofthe remaining substantially two thirds of the sheet is formed fromvirgin interior glass (22). Two other portions of the glass flow (24)which are symmetrically offset from the front surface at an angle ofapproximately 45 degrees end up forming the near end unusable edgesection (25) at the inlet end of the sheet. Another portion (26)centered at the back surface at an angle of approximately 180 degreesproceeds to the far end unusable edge section (27).

FIGS. 3A and 3B show an embodiment of the glass sheet forming apparatus(31) with an inflow pipe (8), a flow distribution device (32) (which isa subject of this invention) located at the trough inlet surface, andthe glass sheet forming apparatus body (9). The flow distribution device(32) interrupts the glass surface flow and diverts it to the surface inthe edge of the sheet. Glass from the center of the downcomer pipe flowstream then comes to the surface of the forming trough to form thesurface of the useable portion of the glass sheet (11). Note that ten totwenty percent of the sheet at each edge is normally unusable forvarious reasons.

FIGS. 4A and 4B show an alternative embodiment of the glass sheetforming apparatus (41), which performs the same function as theembodiment in FIG. 3 except that the surface flow distribution device(42) is located under the surface of the glass (10) and redistributesthe surface flow in a more subtle but equally effective manner. Theglass flow (10) that forms the unusable inlet edge of the sheet, flowsthrough the center slot (43) in the flow distribution device (42). Theglass (which flows through this center slot) is the glass that has beenin proximity to the front surface of the downcomer pipe. Glass from thecenter of the downcomer pipe then flows to the trough surface to formthe surface of the useable portion of the sheet (11). Other glass thatflows in proximity to the surface of the downcomer pipe remainssubmerged.

FIGS. 5A through 5C illustrate where the glass (10) flowing in thedowncomer feed pipe (7) ends up in the formed glass sheet for theinventions described in FIGS. 3 and 4. The glass flow to the center ofthe sheet (21) is virtually identical to that in the prior art. However,the flow (52) which forms the outside surface of the formed glass sheetdoes not flow in proximity to the front surface of the downcomer pipe(7). The two portions of the glass flow (24) which are symmetricallyoffset from the front surface at an angle of approximately 45 degreesand which end up forming the unusable edge section (25) at the inlet endof the sheet are substantially unaffected, as is the glass flow (26)which ends up in the unusable edge section (27).

FIG. 6 is an embodiment that shows the axis of the bowl (66) inclined atan angle such that the main process stream passes through the front ofthe bowl. This active flow (60) entrains the surface glass (61),overcoming the surface tension forces that would normally create aquiescent zone of glass flow located at the bowl nose (FIG. 8). A needle(13) is present to stop glass flow.

FIGS. 7A through 7D show an embodiment of the present invention where acrossways motion of the glass in the bowl (76) is facilitated by feedingthe glass in the pipe coming from the stirring device to the bowl (75),into the side of the bowl (76) at an angle (74) with respect to thecenterline (73) of the forming apparatus (9). This effectively changesthe flow pattern (70) in the bowl such that the quiescent zone normallylocated at the bowl nose (81, FIG. 8) is moved to the side of the bowl(71). Referring back to FIGS. 2A-2C and 5A-5C, depending on the angle(74) of the flow in the bowl with respect to the centerline (73) of theforming apparatus (9), the glass from the quiescent zone (71) ends up ineither the unusable portion of the edges (25), (27) or is submerged inthe center of the glass sheet (21) instead of on the surface of theglass sheet (23). The glass free surface (72) in the bowl is also shown.

FIG. 8 illustrates the prior art with a bowl (6) which shows thequiescent zone (81) of glass that is located at the front of the bowl(6). This glass is kept in place by a combination of low process streamflow (80) at the front of the bowl and surface tension.

FIGS. 9A through 9C show an embodiment of the present invention wherethe bottom end (94) of the downcomer pipe (97) is located substantiallyabove the glass free surface (90) in the forming apparatus inlet pipe(98). The bottom end of the downcomer pipe (97) and the formingapparatus inlet (98) also have a specific size and shape, (95) and (92),respectively. The vertical distance (93) and the size and shape (92) ofthe forming apparatus inlet (98) is specifically designed to minimizeany zone of quiescent or vortex flow in the glass flow path (91). Thus,the molten glass (10) forms a more homogenous sheet (11). This design isdetermined by solution of the fluid flow equations (Navier-StokesEquations) and by experimental tests.

FIGS. 10A through 10C show a downcomer pipe (7) submerged in the moltenglass surface (100) in the forming apparatus inlet pipe (8) as known inthe prior art. There is a quiescent zone (101) between the two pipes (7)and (8). The glass flow path (103) produces an annular vortex (102) ofglass between the downcomer pipe (7) and the trough inlet pipe (8). Thevortex exchanges little material with the main process stream exceptduring flow transients at which time it produces defects in the glasssheet. The minimization of the vortex between the downcomer pipe (7) andthe inlet pipe (8) requires accurate centering of the two pipes as doesaccurate relative vertical positioning (93) of the two pipes. This is adifficult condition to maintain in production operations. Thesensitivity of the process to the vertical position (93) can be reducedby the embodiments discussed relative to FIGS. 33 through 42.

Referring back to FIGS. 2A through 2C, and also to FIGS. 33A through42B, a preferred embodiment of the present invention substantiallyalters the vortex flow at the inlet of the sheet glass forming apparatusto improve quality. The vortex flow between the downcomer pipe and theinlet pipe at the inlet to the apparatus is altered from a single vortexto multiple vortices.

The streamlines of glass flow shown in FIGS. 33B, 34B, 36B, 37B, 39B,41B, and 42B in this application are calculated using a technologytermed “computational fluid dynamics”, which uses computers to predictthe motion of gases and liquids. The particular product used for thecalculations herein was CFD2000, which is one of several products thatare commercially available.

FIGS. 10A through 10C and FIGS. 33A and 33B show the prior art, wherethe downcomer pipe (7) has a flat bottom (94) which is immersed belowthe free surface of the glass (100) in the inlet pipe (8). There is aquiescent zone (101) between the two pipes which has a continuous vortex(102) which surrounds the bottom of the downcomer pipe (7).

FIG. 33B shows streamlines of glass flow (330) from this vortex (102) asexperienced in the prior art when the centerline of the downcomer pipe(7) is centered with the centerline of the inlet pipe (8). The flow ofglass (330) from the vortex (102) is evenly distributed around theinside surface of the inlet pipe (8).

FIGS. 34A and 34B show an embodiment of the present invention where thecenterline of the downcomer pipe (7) is not centered with the centerlineof the inlet pipe (8). The single vortex divides into two vortices(342), one on each side of the downcomer pipe (7). The flow in thesevortices is no longer isolated from the main stream of glass flow. Glassenters each vortex where the pipes are closest (343) and exits wherethey are widest apart (344). In addition to the rotational flow of thevortices, there is a migration of flow in the direction to where thepipes are widest apart (344). This migration of flow is not as strong asis discussed in subsequent embodiments of this invention, however, thephenomenon may be used as a method of altering the flow of glass whenused with prior art downcomer and inlet pipes. Direction arrow (349)shows the orientation of the trough from the inlet end to outlet end.The off center orientation of the downcomer pipe (7) relative to theinlet pipe (8) is such that the glass exiting the vortices (340) passesthrough area (26), thus it will flow along the bottom of the trough (9)to the unusable far end (27) of the sheet.

FIGS. 35A, 36A and 36B show an embodiment of the present invention wherethe centerline of the downcomer pipe (357) is centered with thecenterline of the inlet pipe (8), but where the bottom (359) of thedowncomer pipe (357) is cut at an angle to the centerline as illustratedin FIG. 35A. The single vortex divides into two vortices (362), one oneach side of the downcomer pipe (357). The migration of flow in thevortices is toward the downward extending tip (351) of the downcomerpipe. Direction arrow (369) shows the orientation of the trough from theinlet end to outlet end. The orientation of the downward extending tip(351) of the downcomer pipe (357) relative to the inlet pipe (8) is suchthat the glass exiting the vortices (360) passes through area (26), thusit will flow along the bottom of the trough (9) to the far end (27) ofthe sheet.

In FIG. 35A, the bottom (359) of the downcomer pipe (357) is immersedbelow the free surface of glass (100) in the inlet pipe (8). FIG. 35Bshows an additional embodiment of this invention whereby the bottom(359) of the downcomer pipe (357) is partially immersed in the freesurface of glass (100) in the inlet pipe (8). FIG. 35C shows anadditional embodiment of this invention whereby the bottom (359) of thedowncomer pipe (357) is substantially above the free surface of glass(100) in the inlet pipe (8).

FIGS. 37A and 37B show a downcomer pipe (357) with a single downwardextending tip (351) located off center with respect to the inlet pipe(8). The glass exiting the vortices (370) is further concentrated intozone (26).

The altered shape of the bottom end of the downcomer pipe and/or theinlet pipe could also be utilized in combination with the embodimentshown in FIGS. 9A through 9C. In that embodiment, the bottom end (94) ofthe downcomer pipe (97) is located substantially above the glass freesurface (90) in the forming apparatus inlet pipe (98).

FIG. 38A and FIGS. 39A and 39B show an embodiment of the presentinvention where the bottom end (389) of the downcomer pipe (387) isshaped to influence the shape of the vortices in the quiescent zone(101) between the downcomer pipe (387) and the forming apparatus inletpipe (8). The bottom end (389) has a non-flat and non-linear shape. In apreferred embodiment, there are three V shaped downward extensions(391). Direction arrow (399) shows the orientation of the trough fromthe inlet end to outlet end. The three downward extensions (391)effectively divide the single vortex into three sets of two separatevortices (392). The downward extensions are oriented relative to thetrough such that the flow dynamics of the glass exiting the sets ofvortices (392) is directed to the unusable ends of the sheet (25) and(27). Although a V shape or a scallop shape is preferred, any shape thatinduces a vortex, or any downcomer pipe discontinuity would work in thepresent invention.

FIG. 39B shows streamlines of glass flow (394) and (396) from thesethree sets of vortices. Referring also back to FIG. 2, the flow of glass(394) from the two front sets of vortices is concentrated such that itflows to regions (24) in the inlet pipe and thus ends up in the inletend (25) in the sheet. The flow of glass (396) from the back set ofvortices is concentrated such that it flows to region (26) in the inletpipe and thus ends up in the outlet end (27) in the sheet. The downcomerpipe bottom preferably has a V shape with the three extended V shapes(391) coincident with the exit glass flow (394) and (396) from thevortices (392).

In FIG. 38A, the bottom (389) of the downcomer pipe (387) is immersedbelow the free surface of glass (100) in the inlet pipe (8). FIG. 38Bshows an additional embodiment of this invention whereby the bottom(389) of the downcomer pipe (387) is partially immersed in the freesurface of glass (100) in the inlet pipe. (8). FIG. 38C shows anadditional embodiment of this invention whereby the bottom (389) of thedowncomer pipe (387) is substantially above the free surface of glass(100) in the inlet pipe (8). Having the option to either submerge thebottom end (389) in glass, partially submerge the bottom end (389) inglass, or locate the bottom end (389) substantially above the freesurface provides versatility in the control of the size, location, andactivity of the vortices. Additionally, the downcomer pipe (387) may bedisplaced horizontally relative to the inlet pipe (8) or (408) such thatthey are not concentric as shown in FIG. 34A.

FIGS. 40A, 40B, 41A and 41B show an alternative embodiment with amodification to the shape of the inlet pipe. In this embodiment, theinlet pipe (408) is radially extended, in three locations, each adjacentto and corresponding to the three V shapes (391) at the bottom end ofthe downcomer pipe (387). FIG. 40B shows top view of the altered shapeof the inlet pipe (408). This modification to the inlet pipe creates aset of vortices for each extension much like what is created when thedowncomer pipe is off center as discussed earlier.

FIG. 41B shows streamlines of glass flow (414) and (416) from thesethree sets of vortices. Referring also back to FIG. 2, the flow of glass(414) from the two front sets of vortices is concentrated such that itflows to regions (24) in the inlet pipe and thus ends up in the inletend (25) in the sheet. The flow of glass (416) from the back set ofvortices is concentrated such that it flows to region (26) in the inletpipe and thus ends up in the outlet end (27) in the sheet. Although themodification to the inlet pipe (408) is only shown with respect to threedownward extensions, the inlet pipe could be similarly modified tocorrespond to either one or two downward extensions in the bottom of thedowncomer pipe.

FIGS. 42A and 42B show an alternative embodiment with a modification tothe shape of the bottom of the downcomer pipe (387) and a modificationto the shape of the inlet pipe (408). FIG. 42B shows streamlines ofglass flow (424) and (426) from these three sets of vortices. Thesimultaneous incorporation of modifications to the downcomer pipe (387)and the inlet pipe (408) further concentrate the streamlines of glassflow (424) and (426) in areas (24) and (26), respectively. Shaping boththe downcomer pipe (387) and the inlet pipe (408) facilitates additionalversatility in the shape and performance of the vortices, so that thevortices may be larger and thus more active.

The altered shape of the bottom end of the downcomer pipe and/or theinlet pipe could also be utilized in combination with the embodimentshown in FIGS. 9A through 9C. In that embodiment, the bottom end (94) ofthe downcomer pipe (97) is located substantially above the glass freesurface (90) in the forming apparatus inlet pipe (98).

Although not shown, the invention also contemplates the use of twodownward extensions. For each extension, or V shape, two vortices areformed. So, a total of four vortices would be formed if two downwardextensions were utilized. The downward extensions would be oriented toconcentrate the flow exiting the vortices in either both zones (24) orzone (26) and one zone (24).

Reducing Degradation of Sheet Glass Forming Apparatus

Referring now to FIGS. 12 through 16, another embodiment of the presentinvention controls the flow distribution of glass on the formingapparatus in a manner such that the degradation of the productionapparatus and the deformation of the forming trough that results fromthermal creep is compensated by thermal control of the glass flowdistribution.

U.S. Pat. No. 3,338,696 relies on a specifically shaped forming troughto distribute the glass in a manner to form a sheet of uniformthickness. The basic shape of this forming trough is described in detailin U.S. Pat. No. 3,338,696. The flow of glass on the sides of theforming apparatus is strongly influenced by surface tension and bodyforces. The sheet glass forming process is conducted at elevatedtemperatures, typically between 1000° C. and 1350° C. At thesetemperatures, the refractory materials used for construction of theforming trough exhibit a property called thermal creep, which isdeformation of the material cause by applied stress. Thus, the troughdeforms under the stress caused by its own weight and the stress causedby the hydrostatic pressure of the glass in the trough.

The materials used in the construction of the other parts of the formingapparatus also degrade (warp, crack, change thermal properties, etc.) inan indeterminate way, which has an adverse effect on thicknessdistribution. The thickness control system of U.S. Pat. No. 3,682,609can compensate for small thickness errors, but it can only redistributethe glass over distances on the order of 5-10 cm. To significantlyeffect thickness distribution over the entire width of the glass sheet,the flow of the molten glass over the weirs must be controlled.

This embodiment of the invention solves this problem by introducing aprecise thermal control system to redistribute the flow of molten glassat the weirs, which is the most critical area of the forming process.This thermal control effectively counteracts the degradation of thesheet forming apparatus which inevitably occurs during a productioncampaign.

FIG. 12A shows the side view of the forming trough (9) with arrowsshowing the flow of molten glass (10) through the forming trough (9) tothe side weirs (115). FIG. 12B shows a section through the center of theforming trough (9) which shows the different zones for the control ofmolten glass (10) as it flows through the forming apparatus. Zone (121)is the flow from the inlet end of the trough to the far end, zone (122)is the flow over the weirs, zone (123) is the flow down the outside ofthe forming trough, and zone (124) is the molten glass (11) being pulledoff the root (116) and cooling into a solid sheet (12). The effect onthe solid glass sheet (12) thickness caused by heating or cooling themolten glass (10) as it passes through each zone is different. Addingenergy to (raising the temperature of) or removing energy from themolten glass (10) as it flows from the inlet end to the far end of theforming trough (9) in zone (121) produces concave or convex sheetthickness profiles respectively. The period of the thickness profilechanges effected in zone (121) is on the order of the length of thetrough.

Changes to the energy flux to the molten glass (10) as it flows over theweirs (115) in zone (122) has a powerful effect on the resultant solidglass sheet thickness distribution. Localized cooling of the glass inzone (122) effectively produces a dam, which has a large effect on glassflow. This is an extremely sensitive zone, and any control strategyother than isothermal must be carefully designed. Zone (123) isimportant to return the glass to a uniform temperature distribution,substantially linear in the longitudinal direction, in order that thedrawing process at the root (116) is consistent. Differential cooling inzone (124) is the object of U.S. Pat. No. 3,682,609 and is effective inmaking small thickness distribution changes. Cooling at givenlongitudinal location affects the thickness at that location in onedirection and conversely to the glass on each side of the location. Theeffect is longitudinal redistribution of the glass over a distance onthe order of centimeters.

FIGS. 13A and 13B show an embodiment of this invention whereby the topand sides of the muffle (132) are shaped more closely to the outsidesurface of the molten glass (10) that is flowing in and on the formingtrough (9). The muffle (132) is heated by heating elements in heatingchamber (131). The primary heat transfer medium in the muffle chamber(113) is radiation. By designing the muffle (132) to conform closely tothe outside shape of the molten glass (10), energy may be directed totargeted areas of the molten glass (10), thereby effecting greatercontrol of temperature distribution. The heating elements in the heatingchamber (131) have adequate power to balance the energy flux to theforming trough (9) and thus create suitable temperature conditions.

FIGS. 14A and 14B show an embodiment of this invention which effectslocalized cooling of the molten glass (10) as it passes over the weirs(115) in zone (122). The muffle (132) configuration of FIGS. 13A and 13Bis used. Air cooling tubes (142), similar in function to those aircooling tubes (141), which are described in U.S. Pat. No. 3,682,609, aredirected at the heating chamber side of the muffle (143) just above themolten glass (10) flowing over the weirs (115). Localized cooling of theglass in this location effectively produces a localized dam, which has asignificant effect on the thickness distribution of the solid glasssheet.

FIGS. 15A and 15B show an embodiment of this invention whereby themulti-chamber muffle (156) is designed with separate heating chambers(151-155) to control the temperature of the molten glass (10) as itspasses through the various individual zones of the forming process.These zones (121-124) are described in FIGS. 12A and 12B. Themulti-chamber muffle (156) has five heating chambers (151-155). Heatingchamber (153), located over the top of the forming trough (9), effectsthe flow of glass from the inlet end to the far end of the formingtrough (9), (zone (121)). The heating chambers (152) and (154) over thetop of the weirs (115) effect the flow over the weirs (115) (zone(122)), and the heating chambers (151) and (155) on each side of thetrough (9) are used to balance the temperature longitudinally (zone(123)). All the heating chambers (151-155) have heating elements withadequate power to balance the energy flux to the forming trough (9) andthus create suitable temperature conditions.

FIGS. 16A and 16B show an embodiment of this invention which affectslocalized cooling to the molten glass (10) as it passes over the weirs(115). This is zone (122) shown in FIG. 12B. The multi-chamber muffle(156) configuration of FIGS. 15A and 15B is used. Specially designedradiant coolers (161), installed in heating chambers (152) and (154),have the ability to selectively cool the heating chamber side of themuffle surface (162) opposite the weirs (115). The radiant cooler hasmultiple adjustments (164) such that the temperature of its bottomsurface can be varied in the longitudinal direction. The distribution ofthe heat transfer between the radiant cooler (161) and the mufflesurface (162) is a function of the distance (163). By varying thedistance (163) between the cooling device (161) and the muffle surface(162), the cooling effect may be attenuated to adjust sensitivity.Although it is not illustrated, the cooling devices (161) arereplaceable during operation. The radiant coolers (161) couldalternately be inserted from the side instead of the top with a suitablechange in the design of the heating chambers (152), (153) and (154).

In an alternative embodiment, the air cooling tubes (142) of FIGS. 14Aand 14B could be used with the muffle (156) design of FIGS. 15A and 15B,and the radiant coolers (161) of FIGS. 16A and 16B could be used withthe muffle (132) configuration of FIGS. 13A and 13B.

Reducing Thickness Variations in the Glass Sheet

Referring to FIGS. 17 through 20, another embodiment of the presentinvention supports and stresses the forming apparatus in a manner suchthat the deformation that results from thermal creep has a minimumeffect on the thickness variation of the glass sheet. This embodimentintroduces a counteracting force to these stresses on the trough in amanner such that the thermal creep which inevitably occurs has a minimumimpact on the glass flow characteristics of the forming trough. Theinvention is designed such that this counteracting force is maintainedthrough an extended period of the production campaign. Thus, sheet glassmay be manufactured for a longer time with the same forming trough.

The refractory materials from which the forming trough and its supportstructure are made have high strength in compression and low strength intension. Like most structural materials, they also change shape whenstressed at high temperature. This embodiment was developed due to thematerial characteristics and how these characteristics affect themanufacturing process.

There are two fundamental concepts in this embodiment of the invention.First, applying a force and/or moment to the ends of the troughcounteracts stress caused by the forces of gravity, thus minimizing theeffect on molten glass flow caused by thermal creep. Second, theinvention uses compression members shaped such that thermal creep, towhich the compression members are also subject, does not substantiallyalter the application of said force and/or moment.

FIGS. 17A and 17B illustrate the typical effects of thermal creep on theshape of the trough. FIG. 17A shows that the forming trough (9) sags inthe middle such that the top of the weirs (115), and the root (116) arenow curved (171) and the trough bottom (117) has a change in curvature(171). This curvature (171) causes the molten glass (10) to no longerflow with constant thickness (172) over the weirs (115). This curvature(171) allows more glass to flow over the middle of the weirs resultingin an uneven sheet thickness distribution. FIG. 17B shows how thehydrostatic force (174) from the molten glass (10) in the forming trough(9) forces the weirs (115) to move apart at the top. This allows moreglass to flow to the middle of the forming trough (9) making thethickness in the middle even greater.

FIGS. 18A through 18D show a sheet glass forming apparatus (180) asknown in the prior art. The forming trough (9) is supported by an inletend supporting block (181) and a far end supporting block (182). Theforming trough (9) is the equivalent of a beam, which is subject to abending stress from its own weight, from the weight of the glass in andon the trough, and from drawing forces. Because of the low tensilestrength of the trough material, a compressive force (183) is applied tothe lower half of the forming trough (9) to force the material at theroot (116) of the forming trough (9) into compression. Typically theinlet end support block (181) is restrained in the longitudinal(horizontal) direction and the compression force (183) is applied to thefar end support block (182). The prior art considers only preventingtension at the root (116) of the forming trough (9), and then only thestress at start-up. Little consideration is made for the effects onstress of the thermal creep of the forming trough (9) and its supportblocks (181) and (182).

FIGS. 19A through 19D show an embodiment of a sheet glass formingapparatus (190) that has shaped end support blocks (191) aid (192). Theinlet end shaped support block (191) is restrained in the longitudinaldirection. A compression force (193) is applied to the far end shapedsupport block (192). The shape of the support block is designed in amanner to produce a force distribution in the forming trough (9) tosubstantially counteract the effect of the weight of the forming trough(9) and the molten glass (10). The applied force (193) is such that allmaterial in the forming trough (9) is under substantially equalcompression stress in the longitudinal direction. This stress causes thethermal creep to occur primarily in the longitudinal direction withlittle of the sagging shown in FIG. 17A. The forming trough (9) getsshorter due to the equal compressive stress in the longitudinaldirection. The shaped support blocks are also subject to thermal creep.The cross section of the shaped support block is the same oversubstantially its entire length with equal compressive stress across itssection. Thus as the shaped support block deforms from thermal creep, itcontinues to apply substantially the same force distribution to theforming trough (9).

FIGS. 20A through 20D show an embodiment of a sheet glass formingapparatus (200) that has four shaped end support blocks (201), (202),(204), and (205). The inlet end has three shaped support blocks (201),(204), and (205), all of which have longitudinal compression forces(206), (207), and (208) applied. A compression force (203) is applied tothe far end shaped support block (202). The shape and loading of thesupport blocks (202) and (203) are designed to the same criteria assupport blocks (191) and (192) in FIGS. 19A-19D. The two upper shapedsupport blocks (204) and (205) are attached to the inlet end of theweirs and are angled such that they exert an additional force on theweirs to counteract the affect of the hydrostatic forces which tend tospread the weirs apart. Although the blocks (204) and (205) are shownwith an inward angle in the figures, they also could be angled outwardwithout deviating from the spirit of the invention.

In a preferred embodiment, short (10-25% of length) transition zones(not shown) are at the trough ends of the shaped support blocks. Inthese transition zones, the cross-section of the shaped support blockwill change from that of the shaped support block to a shape that willsuitably apply the design load to the trough block.

Referring back to FIGS. 17A through 20D, and also to FIGS. 51A through55D, additional embodiments of the present invention support and stressthe forming apparatus in a manner such that the deformation that resultsfrom thermal creep has a minimum effect on the thickness variation ofthe glass sheet. These embodiments introduce a counteracting force tostresses on the trough in a manner such that the thermal creep, whichinevitably occurs, has a minimal impact on the glass flowcharacteristics of the forming trough. The invention is designed suchthat this counteracting force is maintained through an extended periodof the production campaign. Thus, sheet glass may be manufactured for alonger time with the same forming trough.

FIGS. 51A through 51C illustrate the typical effects of thermal creep onthe shape of the trough when the end support blocks impart differentcompression stress in the bottom of the trough. FIG. 51A shows that,with no compression loading, the forming trough (9) sags in the middlesuch that the top of the weirs (115) and the root (116) are now curved(171) and the trough bottom (117) has a change in curvature (171). Thiscurvature (171) causes the molten glass (10) to no longer flow withconstant thickness (172) over the weirs (115). This curvature (171)allows more glass to flow over the middle of the weirs resulting in anuneven sheet thickness distribution. FIG. 51B shows that sagging of thetrough is minimized under the optimum compression loading (515) of thebottom of the trough. FIG. 51C shows that if too much load (516) isapplied to the bottom of the trough the bottom is compressedexcessively, thus producing a convex shape (511) to the trough weirs andbottom.

FIGS. 52A through 52D show a sheet glass forming apparatus (520) thatrepresents prior art. The forming trough (9) is supported by an inletend supporting block (521) and a far end supporting block (522). Theinlet end support block (521) rests on the inlet end structure (523) andis restrained in the longitudinal (horizontal) direction by anadjustment screw (524). The far end support block (522) rests on the farend structure (525), and an far end compression force (526) is appliedto the far end of the trough by support block (522) at surface (527).The force (526) is generated by the far end force motor (528) actingbetween the support block (522) and the far end structure (525). “Forcemotor” as used herein, represents a device that generates asubstantially constant force and is adjustable. For example, the forcemotor may be in the form of an adjustable spring, an air cylinder, ahydraulic cylinder, an electric motor, an adjustment screw, or a weightand lever system. Present practice considers primarily preventingtension at the root (116) of the forming trough (9), and only the stressat start-up.

FIGS. 53A through 53D show the typical shape of a prior art sheet glassforming apparatus (520) resultant from the effects of thermal creepunder the influence of the far end compression force (526). The inletend shaped support block (521) is restrained in the longitudinaldirection by the adjusting screw (524) and applies an inlet endcompression force (536) to surface (537). The trough (9) sits on inletblock (521) at surface (531). The trough (9) sits on far block (522) atsurface (532). The trough inlet pipe (8) is also constrained in thehorizontal direction in order to maintain a seal between the inlet pipe(8) and the trough (9). Thus, the inlet end structure (523) applies anadditional inlet end compression force (538) at surface (539). At thestart of a production campaign the adjusting nut (524) is set such thatthe inlet end force (536) and the far end force (526) are substantiallyequal and opposite. As the production campaign progresses, the trough(9) starts to deform via thermal creep under the influence of gravityand the applied horizontal compression forces. The distance betweensurfaces (527) and (537) becomes less. As this occurs, the force (538)at surface (539) between the inlet pipe (8) and the inlet end structure(523) becomes greater as it absorbs a portion of the horizontal forcefrom the force (526) applied at the far end of the trough. As the sum ofthe horizontal forces must be zero, force (536) at surface (537)decreases. The loading on the trough is no longer symmetrical, thuscontour of the weirs (115), trough bottom (117), and root (116) take anS-shape (533). The S-shape is typical, however, it can varysignificantly depending on the rigidity of the inlet end structure (523)at surfaces (537) and (539).

There are three different groups of embodiments to correct the designshortcomings of the prior art.

The first embodiment, which may be used with a prior art apparatus,periodically adjusts the inlet end adjustment screw (524) to compensatefor the shortening of the distance between surfaces (527) and (537). Thehorizontal displacement of surface (537) may be measured and acorresponding adjustment of the screw (524) is made. Ideally thehorizontal displacement of surface (537) will be halved and there willbe a corresponding negative horizontal displacement of surface (527).The torque on the adjustment screw (524) may also be monitored, however,the friction at surface (521) will degrade the accuracy of torque as aindicator of the force (536) actually applied to the trough (9). Thisembodiment of the invention is counter intuitive as adjusting theadjustment screw (524) in a direction to lessen the integrity of theglass seal between the inlet pipe (8) and the trough (9) will makeoperating personnel nervous.

A second and better embodiment of the invention, which requires a designand construction change to the forming apparatus, controls the relativehorizontal rigidity and resistance to thermal creep of the inlet endstructure (523) at points (537) and (539). This embodiment changes thedesign to make the inlet end structure (523), which supports theadjusting bolt or screw (524) and applies the force (536) to the troughat point (537), substantially more resistant to thermal creep than thestructure at point (539), which effects a glass seal between the inletpipe (8) and the trough (9). This causes the force (536) at the inletface of the trough (537) to remain more equal to the far end force(526), thus maintaining the primary compression force near the root(116) of the trough (9). The force at point (539) rises but is limitedsuch that it does not rise significantly above the level required toeffect a satisfactory glass seal between the inlet pipe (8) and thetrough (9).

In this embodiment, the refractory structure (523) at the point (539),that comprises the support force (538) for the glass seal, is designedto accommodate a sufficient amount of thermal creep such that the inletpipe (8) and point (539) move in the same direction as the far end ofthe trough (527), thus allowing the adjustment bolt (524) to continue toprovide a compressive force to the bottom of the trough inlet end atpoint (537) as points (527) and (537) move closer together. Thestructure (523) at point (539) must be sufficiently stiff to maintainthe glass seal, yet yield enough such that force (538) does not increasesignificantly, thus subtracting from force (536).

The third and best improvement over the prior art is a group ofembodiments, which represent a substantial design change, and, with theuse of multiple force motors and low friction design concepts, enablethe compressive forces (526) and (536) at the root (116) of the trough(9) to remain consistently at the desired levels throughout theproduction campaign.

This group of embodiments applies a substantially equal and oppositecompression force to each end of the bottom of the trough to negate thethermal creep, thus minimizing the sagging of the trough and its effecton molten glass flow. This is accomplished in a manner which has lowfrictional forces over the life of the production campaign.

FIGS. 54A through 54D show a sheet glass forming apparatus (540), wherethe weight of the trough is supported at the inlet end by the inlet endstructure (523) at surface (541). Additionally, it is constrainedhorizontally by a small compression force (538) at surface (539). Theweight of the trough is supported at the far end by the far endstructure (525) at surface (542). Surface (542) is designed to have verylow friction in the horizontal direction, thus contributing negligibleforce in the horizontal direction. The inlet end compression force (536)is applied to the bottom of the trough by a compression block (543),which is designed to have low friction in the direction of the appliedforce, designated as “free-floating” herein. The inlet end compressionforce (536) is generated by the inlet end force motor (548). The far endcompression force (526) is applied to the bottom of the trough by thefree-floating compression block (544). The far end compression force(526) is generated by the far end force motor (528). The far endcompression force (526) must be slightly greater than the inlet endcompression forces (536) to compensate for the inlet pipe compressionforce (538). The trough bottom compression forces (526) and (536) areapplied friction free and can be maintained at the same and/or anypreprogrammed level throughout a production campaign. Note that thecross-sectional shape of the inlet end compression block (543) and thefar end compression block (544) is the same as that of the trough wherethe forces are applied to the trough. This minimizes the stressconcentrations where the forces are applied. There are keys (545)between the compression blocks and the trough to insure connectalignment of the compression blocks to the trough. These keys may berectangular, as shown at the inlet end, or circular as shown at the farend. Also shown are chamfers (546) between the pointed end of the root(116) and the bottom edge of the compression blocks (543) and (544) ateach end of the trough. These chamfers (546) are not in the glasscontact area of the trough. The chamfers (546) reduce a small area oftension stress at this location.

FIGS. 55A through 55D show a sheet glass forming apparatus (550), wherethe weight of the trough is supported at the inlet end by the inlet endstructure (523) at surface (541). Additionally, it is constrainedhorizontally by a small compression force (538) at surface (539). Theweight of the trough is supported at the far end by the free-floatingfar end compression block (554), which has been set at an angle (551) tothe horizontal. Although not shown, either (or both) free-floatingcompression block (553) or (554) can be set at an angle to thehorizontal. If both compression blocks (553) or (554) are at an angle,the compression force at the bottom of the trough is generated by theweight of the trough.

The compression force (556) on the trough contains vertical andhorizontal components such that the horizontal compression force (526)equals the weight of the trough at the far end divided by the tangent ofthe angle (551). The compression force (556) is generated by the forcemotor (558) acting on the far end structure (555). The inlet endcompression force (536) is applied to the bottom of the trough by thefree-floating compression block (553). The inlet end compression force(536) is generated by the inlet end force motor (558). The compressionforce (556) must be such that the far end compression force (526) isslightly greater than the inlet end compression forces (536) tocompensate for the inlet pipe compression force (538). The trough bottomcompression forces (556) and (536) are applied friction free and can bemaintained at the same and/or any preprogrammed level throughout aproduction campaign. The compression block (554) is keyed to the troughat points (557) to insure correct alignment of the compression blocks tothe trough. A boss (559) is preferably formed into the inlet end of thetrough to better distribute the inlet end compression force (556) intothe trough. The boss (559) is not in the glass contact area of thetrough and therefore has no adverse effect on the glass flow. The inletend compression block (553) is reshaped to have the same cross-sectionshape as the boss (559) where the two contact each other.

FIGS. 56A through 56D is an embodiment of a sheet glass formingapparatus (560) much like that shown in FIGS. 54A through 54D exceptthat a force motor (568) is added to far end structure (565) at the topend of the far end of the trough (10). The force motor (568) adds aforce (566) that is equal and opposite to the glass sealing force (538).This allows the compression force (526) on the far end to be equal tothe compression force (536) on the inlet end.

In FIGS. 56A through 56D, forces (538) and (566) as well as forces (536)and (526) are not precisely equal and opposite. Since the trough isheavier and has a larger cross-section on the inlet end, these forcesmay be adjusted to provide a couple to compensate for the effect of thisadditional weight and cross-section on thermal creep.

FIGS. 57A through 57D show an embodiment of a sheet glass formingapparatus (570) whereby the inlet end adjusting screw (524) is retainedand a force motor (578) is added to the inlet pipe at surface (539) togenerate a constant sealing force (538) for the glass seal between theinlet pipe (8) and the trough (10).

FIG. 58A through 58D show an embodiment of a sheet glass formingapparatus (580) where the forces (526) and (536) are equal by design.The inlet end structure (583) supports the weight of the trough atsurface (541). The far end structure (585) supports the weight of thetrough at surface (542). The cage (588) has a low friction support (589)such that it is free to move in the horizontal direction. Some examplesfor the support (589) include, but are not limited to, anti-frictionbearings, suspension cables, flexures. The cage (588) connects to andapplies a force to the inlet end compression block (543) and mounts thefar end force motor such as to apply all equal and opposite force to thefar end compression block (544).

Effects of Surface Tension on the Sheet

In an alternative embodiment of the invention, the width and the angleof the inverted slope of the forming wedge may be changed to alter theeffect of surface tension and body forces on the narrowing of the sheet.In addition, the width and the inverted slope angle may be increased tomake the structure stiffer and thus more resistant to thermal creep.

FIGS. 21A through 21G show the prior art shape of the forming trough.The cross-section of the wedge shaped portion, FIGS. 21C through 21G, isuniform over the entire useable length of the trough. The width of thetrough (211) and the angle of the inverted slope (210) are identical ateach section. As the molten glass (10) flows down the vertical portion(211) of the forming wedge (9), the surface tension and body forces havea minimal effect on the sheet width (212), whereas, when the moltenglass (10) flows vertically down the inverted slope portion (210) of theforming wedge, the surface tension and body forces act to make the sheetnarrower (213).

FIGS. 22A through 22G show an identical width of the trough (211) overits entire length, whereas the angle of the inverted slope (210) is thesame in the center of the trough (FIGS. 21D-21F) and the angle of theinverted slope (220) at each end is reduced. This reduced inverted slope(220) has a counterbalancing effect on the surface tension and bodyforce stresses and thus reduces the narrowing of the sheet (223).

FIGS. 23A through 23G show the width of the trough (211) and the angleof the inverted slope (210) being the same in the center of the trough(FIGS. 21D through 21F and FIGS. 22D through 22F), whereas, the width ofthe trough (231) and the angle of the inverted slope (230) at each endare reduced. This reduced width (231) and inverted slope (230) have acounterbalancing effect on the surface tension and body force stressesover the effect of FIGS. 22A through 22G and thus further reduces thenarrowing of the sheet (233).

FIGS. 24A through 24G show another embodiment of this invention, whereinthe width of the trough (211) and (231) and the angle of the invertedslope (210) and (230) are the same as in the embodiment of FIGS. 23Athrough 23G except that the angle of the inverted slope (240) at thecenter of the trough, FIG. 24E is substantially greater than the otherinverted slopes (210) and (230). This greater angle increases thesection modulus of the structure making it stiffer and thus less proneto thermal creep. Keeping the configuration of the ends the same asFIGS. 23A through 23G has substantially the same effect on the surfacetension and body force stresses as FIGS. 23A through 23G and thus haslittle effect on the narrowing of the sheet (243).

Reducing Surface Tension with a Bead Guide

FIGS. 43A through 43G show the prior art shape of the wedge shapedforming device (9). The outside cross-section of the wedge shapedportion, FIGS. 43C through 43G, is uniform over the entire useablelength of the trough. The angle of the inverted slope (430) and thewidth of the trough (431) are identical at each section. As molten glass(10) flows vertically down on the inverted slope portion (433) of theforming wedge (9), the surface tension and body forces act to make thesheet narrower, (432) to (433), at the forming wedge bottom (5). Whenthe sheet leaves the bottom of the trough (116) it contracts further,(433) to (434), in the longitudinal direction. This contraction causesthe glass to form large thick beads (435) at each end as shown in FIG.43B.

FIGS. 44A through 44G show an embodiment of this invention with a beadguide (446) located below the bottom of the trough. The glass width(433) at the bottom (116) of the inverted slope portion of the wedge isnot affected. When the glass leaves the trough bottom it contractsfurther, (433) to (447), until it reaches the bead guide (446). Thewidth, (447) to (448), remains substantially constant in the region ofthe bead guide (446) but further contracts, (448) to (444), beneath thebead guide. When the glass sheet (12) is moving past the bead guide(446), the center of the sheet is solidifying, thus nullifying thesurface tension effects in this region. The subsequent sheet width(444), with a bead guide, shown in FIG. 44A, is greater than the sheetwidth (434), shown in FIG. 43A, without the bead guide (446). The beads(445) shown in FIG. 44B (with a bead guide (446)) are smaller than thebeads (435) shown in FIG. 43B (without a bead guide).

In an alternative embodiment of the invention, the width and the angleof the inverted slope of the forming wedge is changed to alter theeffect of surface tension and body forces on the narrowing of the sheet.In addition, the width and the inverted slope angle may be increased tomake the structure stiffer and thus more resistant to thermal creep. Abead guide is added to the apparatus to reduce the size of the beads ateach end of the sheet.

FIGS. 45A through 45G show an embodiment of the present invention withan altered trough design. In this embodiment the width of the trough(431) is identical over its entire length, the angle of the invertedslope (430) is the same in the center of the trough (FIGS. 45D through45F) whereas the angle of the inverted slope (450) at each end (FIGS.45C and 45G) is reduced. The wedge shaped forming trough (9) has abottom edge (116) which has a concave down shape (459) at each endprimarily to facilitate the reduced angle of slope. The reduced invertedslope (450) introduces a gravitational force which has a countermandingeffect on the surface tension and body force stresses in the center ofthe glass flow stream and thus reduces the narrowing of the sheet, (432)to (453), to substantially zero as it flows down the inverted slope ofthe trough.

In a preferred embodiment, the angles of the slopes (430) and (450) arein the range of 10° to 25°, with the angles (430) generally being largerthan the angles (450). This range is only provided as an example, andother angles are also contemplated by the present invention.

When the glass leaves the trough bottom it contracts further, (453) to(457), until it reaches the bead guide (456). The bead guide in thisembodiment is preferably a rotating disk. The rotation of the diskimparts a stretching force to the glass sheet, which actually increasesthe width of the sheet, (457) to (458), in the region of the bead guide(456). The sheet further contracts, (458) to (454), beneath the beadguide. When the glass sheet (12) is moving past the bead guide (456) thecenter of the sheet is solidifying, thus nullifying the surface tensioneffects in this region. The subsequent sheet width (454) issubstantially the same as the width on the side of trough (432) and thewidth (453) at the bottom of the trough. With suitable development ofthe size, shape, location, rotation speed, and heating of the bead guide(456), the beads (455) shown in FIG. 45B are substantially eliminated,thus the beads have the same or lesser thickness than the useableportion of the sheet.

FIGS. 46A through 46D illustrate various configurations of the beadguide. FIG. 46A shows a simple wedge shaped device (461), which isattached to an X-Y-Z adjusting mechanism (463). The X-Y-Z mechanism(463) is a mechanism that moves the bead guide (461). The mechanism(463) allows the bead guide (461) to be moved around and adjusted asneeded. The glass attaches itself to the front surface (462) of thewedge shaped device (461). FIG. 46B shows a bead guide with the sameshape as that in FIG. 46A, but with provisions for heating by passingelectrical energy through a platinum sheet (464) wrapped around thewedge shaped portion of the bead guide (461). Provision for electricalconnections (465) are attached to the platinum sheet (464). The beadguide in FIG. 46C is similar to that in FIG. 46A but with a rounded top(466). FIG. 46D shows the bottom of the bead guide angled (467) awayfrom the center of the sheet. The exact shape of a bead guide for aspecific manufacturing process is preferably developed by simulation andby test in operation.

FIGS. 47A through 47B illustrate an active bead guide, which includes arotating disk (471), the rotation of which actively pulls the glass fromthe center of the sheet toward the bead area. This bead guide is alsoshown in FIG. 45A. FIG. 47A illustrates a system whereby the rotatingdisk (471) is supported from the end of the chamber containing the sheetforming apparatus. FIG. 47B illustrates a system where the rotating diskis supported by a shaft (472) inserted in through a side of the chambercontaining the sheet forming apparatus. In both cases the rotating diskis attached to a shaft (472), which connects it to the rotating driveand the X-Y-Z adjusting mechanism (473).

Producing a Flat Sheet

U.S. Pat. No. 3,338,696 considers only the glass flow in the formingtrough and assumes that the drawn glass from the bottom of the formingtrough will be of uniform thickness and flatness because of the uniformthickness of the flow of glass to the critical point of solidification.In practice, glass must be preferentially cooled across its width tocreate forming stresses during solidification that create a flat sheet.The present invention alters the forming stresses and coolingdistribution such that the formed sheet is inherently flat.

FIGS. 25A through 25D show an embodiment of this invention where theshape of the bottom of the forming wedge (116) is not straight but isformed convex upward (250). This causes the glass that is drawn from thecenter of the forming wedge (251) to cool faster than the glass drawnfrom each edge (252) of the forming wedge. The strategy is to imposestresses on the partially solidified glass (251) in the center of thesheet to cause the sheet to be flatter, having less warp.

FIGS. 26A through 26D show another embodiment of this invention wherethe shape of the bottom of the forming wedge (116) is not straight butis formed convex downward (260). This causes the glass that is drawnfrom the center of the forming wedge (261) to cool slower than thatdrawn from each edge (262) of the forming wedge. The strategy is to holdthe more solidified edges (262) apart, primarily with the edge rollers(110), such that stresses caused by the shrinkage of the partiallysolidified glass (261) in the center of the sheet cause the sheet to beflatter, having less warp.

FIGS. 27A through 27D show an embodiment of this invention where theshape of the bottom of the forming wedge (116) is not straight but hascomplex shape across its width (270). This causes the glass that isdrawn from the forming wedge (9) to have an equivalent cooling profile.The cooling strategy from this configuration would be a combination ofthat shown in FIGS. 25A through 25D and 26A through 26D.

Reducing Air Leakage

U.S. Pat. No. 3,338,696 relies primarily on careful design and matchingof materials to prevent any material cracks and openings. These cracksand openings are the sources of air leakage, for both initial operationand for operation during the course of a manufacturing campaign. Thisembodiment of the invention provides individual pressure balancingtechnology such that even if a leakage path exists at start-up ordevelops during operation, a minimum quantity of air will flow throughthe leakage paths.

The glass sheet is formed by drawing the glass from the bottom of theoverflow forming trough. The molten glass is cooled and is solidified ina carefully controlled manner. The most desirable cooling phenomena isradiation, which cools the glass substantially evenly through its entirethickness. Convective cooling, which cools only the glass surface, isalso a factor. The convective cooling must be minimized as it has adestabilizing effect on the drawing process. The observed phenomena is acyclic variation in sheet thickness as it is drawn. This is termed“pumping” and is a phenomena noted in all glass drawing processes.

The operating temperature of the forming zone of “The Overflow Process”is typically 1250° C. and is at the top of an open bottom chamber,typically 3 meters high, containing an atmosphere of hot air. Because ofthe approximately 3 meter column of high temperature air, the atmospherein the zone where the sheet is formed has a pressure higher than thepressure outside of the forming apparatus. Therefore, any crack oropening creates an airflow path whereby air flows into the open bottomof the chamber, up the chamber and out the cracks or openings. Thisleakage substantially increases the convective cooling in the formingzone and subsequently produces a cyclic variation in the sheetthickness.

For air to flow through an opening there must be a difference inpressure from one side of the opening to the other. This inventioninvolves adjusting the internal pressure in each of the major componentsof the forming apparatus such that the pressure difference across anyleakage path to the forming zone is essentially zero. Therefore, if anopening either exists or develops, no air leakage will occur as there isno differential pressure to force airflow.

The air pressure in the internal chambers of the forming system issubstantially higher than the ambient pressure in the factory. This isbecause of the low density of the heated air contained in the formingsystem. This elevated pressure forces the internal air to leak throughany openings or cracks in the membranes which separate the forming zonesfrom the heating and cooling zones. The leakage can be minimized byequalizing the air pressure on each side of any leakage path.

Referring now to FIGS. 28A through 32B, FIGS. 28A and 28B show coolingof the glass as it transitions from the molten state to the solid state.This process must be carefully controlled. This cooling process startson the lower part of the forming apparatus (9) just above the root(116), in the muffle zone (280), continues as the molten glass sheet(11) passes through the muffle door zone (114), and is substantiallysolidified by the time it leaves the transition zone (281). Thecontrolled cooling process continues in the annealer and pulling machinezone (282) to relieve internal stress in the solidified glass sheet(12).

Four embodiments for controlling forming chamber pressure differentialsare shown in FIGS. 29A through 32B. These are a) adding flow topressurize, b) restricting outflow, c) flowing to a vacuum, and d)encasement by a pressurizing chamber, respectively. Any of these controlmethods may be used to control the pressure in either the muffle zone(280), the muffle door zone (114), the transition zone (281), or theannealing and pulling machine zone (282) depending on unique designrequirements. The critically important objective, however, is toequalize the pressure on each side of the membrane separating either thefactory atmosphere or a heating zone or a cooling zone from the formingchamber. This invention also applies to implementations of “The OverflowProcess” where the gas in the forming chamber is a gas other than air,i.e. nitrogen, etc.

More specifically, FIGS. 29A and 29B show an embodiment of the mufflezone (280) which shows air (290), which is preferably preheated,introduced into the muffle heating chamber (131) to make the pressure inthe heating chamber (131) equal to that in the adjacent forming chamber(113). The wall (132) separating the two chambers in the muffle isnormally constructed of many pieces and is therefore susceptible torandom leaks. Equalizing the pressure between the two chambers minimizesthe leakage flow.

FIGS. 30A and 30B show an embodiment of the muffle door zone (114),which includes an exit restriction (300) to the flow of air exiting eachmuffle door (301) to factory ambient pressure. The size of thisrestriction is varied to regulate the pressure inside the muffle door(302) equal to that in the pressure in the adjacent forming chamber(303). The flow of air into the muffle doors (301) through the coolingtubes (141) would normally be adequate to overcome any leakage paths andthus raise the muffle door internal pressure (302) to that of theadjacent forming chamber pressure (303).

FIGS. 31A and 31B show an embodiment of the transition zone (281), whichhas the cooling air at elevated pressure (310) entering the coolingchamber (311) and exiting (312) each of the transition coolers (313)into a regulated vacuum source (314). The large volume of air requiredfor cooling in the transition zone would normally raise the pressure inthe transition cooling chamber (311) above that of the adjacent formingchamber (315). A vacuum source (314) is therefore required to lower thepressure and is adjusted to equalize the pressure in the transitioncooling chamber (311) to the pressure in the adjacent forming chamber(315).

FIGS. 32A and 32B show an embodiment of the annealer and pulling machinezone (282), which includes a pair of pressure balancing chambers (320)on each end of the annealer and pulling machine (282). The pressure inthe balancing chamber (321) is adjusted to be equal to the pressure inthe annealing chamber (322). A chamber at each end was chosen becausethe bearings and adjustment mechanisms for the pulling rollers (111) areon the ends. Alternate configurations would be one single pressurebalancing chamber (320) encasing the entire annealer and pulling machine(282) or a multitude of individual pressure balancing chambers (320) aswould be required by particular design considerations.

Placement of Thermocouples in the Incoming Glass Stream and in theTrough

In the process shown in FIGS. 1, 2A through 2C and 11A through 11B, anddiscussed above, it is normal practice to construct the entire glasscontact surface from the stirrer to the trough from sheets of platinum.These sheets are welded together and the welds carefully ground smooth.Discontinuities in the platinum surface adversely effect the glassflowing in proximity to the discontinuity and result in defects in theglass sheet. Additionally it is normal practice that no thermocouplesare permitted to mar this carefully finished internal surface. Thethermocouples which are required for the control of the process aretypically either placed against the outside surface of the platinum orwelded to tabs which are welded to the outside surface. The actualtemperature of the glass stream is not measured and those skilled in theart of temperature measurement know the inaccuracies inherent in thispractice. The rule of thumb, proven in practice, for measuring a processstream temperature is to immerse the thermocouple into the processstream at least four times its diameter. Typically a hollow cylindricalprotrusion, made of platinum and with a length four times its diameter,is welded into the side of the platinum pipe which contains the flowingglass. The thermocouple is inserted into this protective cylinder andcontacts its closed end, which is in intimate contact with the flowingglass stream.

A preferred embodiment of the present invention immerses thermocouplesdirectly in the process stream to accurately measure the glasstemperature in the bowl inlet pipe, the bowl, the downcomer pipe and thetrough inlet pipe, thus facilitating accurate control of glasstemperature and subsequently glass flow. These thermocouples are placedin locations such that they will not have an adverse effect on the sheetglass quality. The glass passing in proximity to these thermocouples isthe glass that forms the unusable edges of the sheet. Typical glassthermocouples are made from platinum wire or platinum/10% rhodium wire.These thermocouples are often fabricated on site, or optionallypurchased from suppliers including, but not limited to, Engelhard-Clalor Johnson Matthey.

FIGS. 48A through 48E show an embodiment of the present invention. Thethermocouples are preferably placed in one or more of the followinglocations: the bowl inlet pipe, the bowl, the downcomer pipe, a bottomcenterline of the trough, and the trough inlet pipe. They are preferablyplaced in locations where the glass defects caused by the flow of glassin proximity to the thermocouples end up in the unusable beads at eachend of the formed sheet. These thermocouples are immersed in the glassstream. Thermocouples (481), (483), (485) and (487) are located suchthat any glass streaks (cord) produced by their disturbance of the glasspassing in proximity forms part of the unusable glass bead (27) at thefar end of the formed sheet. The offset angle of thermocouples (481),(483), (485) and (487) from the centerline preferably ranges from 150 to210 degrees, and is optimally 180 degrees for a symmetrical troughdesign. Thermocouples (482), (484), (486) and (488) are located suchthat any glass streaks (cord) produced by their disturbance of the glasspassing in proximity forms part of the unusable glass bead (25) at theinlet end of the formed sheet. In a preferred embodiment, the offsetangle of thermocouples (482), (484), (486) and (488) from the centerlinepreferably ranges from 30 to 60 degrees, and is optimally 45 degrees;however, the optimal angle may vary considerably based on the design ofthe bowl inlet pipe, the bowl, the inlet pipe and the configuration ofthe inlet end of the trough.

An exception to these defined angles occurs if the bowl inlet pipe (5)is not directly aligned with the trough centerline (9). The optimalangles for thermocouples (487) and (488) are rotated and must bereoriented for this special case.

FIGS. 49A and 49B show the placement of thermocouples (491), seven inthis example, on the bottom longitudinal centerline (492) of the trough.The number of thermocouples is determined by the required informationfor measurement and/or control. The thermocouples (491) are optionallyimbedded in the trough floor by grinding slots in the trough bottom,placing a matrix of thermocouples in the slots and then sealing themwith a suitable cement. Imbedding the thermocouples (491) in therefractory floor of the trough is not a precise measurement of the glasstemperature, however, it is far superior to a measurement inferringglass temperature by measuring the temperature of the air in the troughmuffle chamber (113).

Alternately, as shown in FIG. 50A through 50D, an instrumentationassembly (503) which contains multiple thermocouples (501) may besecured to the trough floor at its longitudinal centerline (502). Thedesign of this assembly (503) is such that the thermocouples (501)project into the glass stream and give a more accurate measurement. Thecross-section of the assembly (503) is preferably uniform in thelongitudinal direction so as not to influence the flow of glass to theweirs (115). In either design, the lateral location of the thermocouples(491), (501) is substantially on the bottom longitudinal centerline(492), (502) such that any glass streaks (cord) produced by theirdisturbance of the glass passing in proximity forms part of the unusableglass bead (27) at the far end of the formed sheet.

Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

1-46. (canceled)
 47. An improved apparatus for forming sheet glass,wherein the apparatus includes a trough inflow pipe for deliveringmolten glass, a trough for receiving molten glass from the inflow pipethat has sides attached to a wedged shaped sheet forming structure thathas downwardly sloping sides converging at the bottom of the wedge suchthat a glass sheet is formed when molten glass flows over the sides ofthe trough, down the downwardly sloping sides of the wedged shaped sheetforming structure and meets at the bottom of wedge, and wherein theimprovement comprises: at least one thermocouple that measures atemperature of the molten glass, wherein the thermocouple is immersed inat least one location directly in a path of molten glass flow, andwherein the location of the thermocouple is such that the thermocoupledoes not have an adverse effect on a quality of a finished glass sheet.48. The apparatus of claim 47, wherein the apparatus further comprises abowl inlet pipe for receiving molten glass from a stirring device, abowl for receiving the molten glass from the bowl inlet pipe, and adowncomer pipe for receiving the molten glass from the bowl, wherein thedowncomer pipe delivers molten glass to the trough inflow pipe, whereinthe location of the thermocouple is selected from the group consistingof: a) the bowl inlet pipe; b) the bowl; c) the downcomer pipe; d) thetrough inflow pipe; e) the bottom centerline of the trough; and f) anycombination of a) through e).
 49. The apparatus of claim 47, whereinthermocouples are placed in one or more locations where at least oneglass defect caused by a flow of glass in proximity of the thermocouplesends up in at least one unusable bead at each end of the formed glasssheet.
 50. The apparatus of claim 47, wherein an offset angle of atleast one thermocouple from a centerline plane of the apparatus rangesfrom 150 to 210 degrees.
 51. The apparatus of claim 50, wherein theoffset angle of at least one thermocouple from a centerline plane of theapparatus is approximately 180 degrees.
 52. The apparatus of claim 47,wherein an offset angle of at least one thermocouple from a centerlineplane of the apparatus ranges from 30 to 60 degrees.
 53. The apparatusof claim 52, wherein the offset angle of at least one thermocouple froma centerline plane of the apparatus is 45 degrees.
 54. The apparatus ofclaim 47, wherein the thermocouple is located on a bottom longitudinalcenterline of the trough.
 55. The apparatus of claim 47, wherein a thethermocouple is imbedded in a floor of the trough.
 56. The apparatus ofclaim 55, wherein imbedding the thermocouple in the floor of the troughcomprises the steps of: a) placing at least one slot in the floor of thetrough; b) placing a thermocouple in each of the slots; and c) sealingthe thermocouples into the floor of the trough.
 57. The apparatus ofclaim 47, wherein the thermocouple is included in an instrumentationassembly.
 58. The apparatus of claim 57, wherein the instrumentationassembly is secured to a floor of the trough at a longitudinalcenterline of the floor.
 59. The apparatus of claim 57, wherein thethermocouple projects into a stream of glass.
 60. The apparatus of claim57, wherein a cross-section of the instrumentation assembly is uniformin a longitudinal direction.
 61. A method for manufacturing glass sheetsusing an apparatus that includes a trough for receiving molten glassthat has sides attached to a wedged shaped sheet forming structure thathas downwardly sloping sides converging at the bottom of the wedge suchthat a glass sheet is formed when molten glass flows over the sides ofthe trough, down the downwardly sloping sides of the wedged shaped sheetforming structure and meets at the bottom of wedge, wherein the methodcomprises the step of: measuring a temperature of the molten glass asthe molten glass travels through the apparatus, wherein the temperatureis measured by at least one thermocouple, wherein the thermocouple isimmersed in at least one location directly in a path of molten glassflow, and wherein the location of the thermocouple is such that thethermocouple does not have an adverse effect on a quality of a finishedglass sheet.
 62. The method of claim 61, wherein the apparatus furthercomprises a bowl inlet pipe for receiving molten glass from a stirringdevice, a bowl for receiving the molten glass from the bowl inlet pipe,a downcomer pipe for receiving the molten glass from the bowl, and atrough inflow pipe for receiving the molten glass from the downcomerpipe, wherein the trough inflow pipe delivers molten glass to thetrough, wherein the location of the thermocouple is selected from thegroup consisting of: a) the bowl inlet pipe; b) the bowl; c) thedowncomer pipe; d) the trough inflow pipe; e) a bottom centerline of thetrough; and f) any combination of a) through e).
 63. The method of claim61, wherein thermocouples are placed in one or more locations where atleast one glass defect caused by a flow of glass in proximity of thethermocouple ends up in at least one unusable bead at either end of theformed glass sheet.
 64. The method of claim 61, wherein an offset angleof at least one thermocouple from a centerline plane of the apparatusranges from 150 to 210 degrees.
 65. The method of claim 64, wherein theoffset angle of at least one thermocouple from a centerline plane of theapparatus is approximately 180 degrees.
 66. The method of claim 61,wherein an offset angle of at least one thermocouple from a centerlineplane of the apparatus ranges from 30 to 60 degrees.
 67. The method ofclaim 66, wherein the offset angle of at least one thermocouple from acenterline plane of the apparatus is 45 degrees.
 68. The method of claim61, wherein the thermocouple is located on a bottom longitudinalcenterline of the trough.
 69. The method of claim 61, further comprisingthe step of imbedding at least one thermocouple in a floor of thetrough.
 70. The method of claim 69, wherein the imbedding step comprisesthe substeps of: a) placing at least one slot in the floor of thetrough; b) placing a thermocouple in each of the slots; and c) sealingthe thermocouples into the floor of the trough.
 71. The method of claim61, wherein the thermocouple is included in an instrumentation assembly.72. The method of claim 71, wherein the method further comprises thestep of securing the instrumentation assembly to a floor of the troughat a longitudinal centerline of the floor.
 73. The method of claim 71,wherein the thermocouple projects into a stream of glass.
 74. The methodof claim 71, wherein a cross-section of the instrumentation assembly isuniform in a longitudinal direction.
 75. An improved apparatus forforming sheet glass, wherein the apparatus includes a trough forreceiving molten glass that has sides attached to a wedged shaped sheetforming structure that has downwardly sloping sides converging at thebottom of the wedge such that a glass sheet is formed when molten glassflows over the sides of the trough, down the downwardly sloping sides ofthe wedged shaped sheet forming structure and meets at the bottom ofwedge, and wherein the improvement comprises: a) at least one inlet endsupport block located at an inlet end of the trough, wherein the inletend support block is positioned at a bottom end of the trough andsupports the trough; b) at least one far end support block located at anopposite end of the trough as the inlet end support block, wherein thefar end support block is positioned at the bottom end of the trough andsupports the trough; c) an inlet end adjustment screw, wherein the inletend adjustment screw restrains the inlet end support block in alongitudinal direction; and d) a far end force motor, wherein the farend force motor applies a force to the far end support block such that abottom of the far end of the trough is deformed, by thermal creep, in alongitudinal direction; wherein the inlet end adjustment screw isperiodically adjusted to maintain an applied force to the inlet endsupport block such that a bottom of the inlet end of the trough isdeformed, by thermal creep, in an opposite longitudinal direction to thedeformation at the far end of the trough; such that any deformation ofthe forming trough that results from thermal creep has a minimal effecton a thickness variation of the glass sheet.
 76. A method formanufacturing glass sheets using an apparatus that includes a trough forreceiving molten glass that has sides attached to a wedged shaped sheetforming structure that has downwardly sloping sides converging at thebottom of the wedge, at least one inlet end support block located at aninlet end of the trough, wherein the inlet end support block ispositioned at a bottom end of the trough and supports the trough, atleast one far end support block located at an opposite end of the troughas the inlet end support block, wherein the far end support block ispositioned at the bottom end of the trough and supports the trough, suchthat a glass sheet is formed when molten glass flows over the sides ofthe trough, down the downwardly sloping sides of the wedged shaped sheetforming structure and meets at the bottom of wedge, wherein the methodcomprises the step of: a) applying a force to the far end support blockto deform, by thermal creep, a bottom of a far end of the trough in alongitudinal direction; and b) applying a force to the inlet end supportblock to deform, by thermal creep, a bottom of the inlet end of thetrough in an opposite longitudinal direction to the deformation of thefar end; such that any deformation of the forming trough that resultsfrom thermal creep has a minimal effect on a thickness variation of theglass sheet.
 77. The method of claim 76, wherein step b) includes thesubstep of periodically adjusting an inlet end adjustment screw tomaintain an applied force to the inlet end support block.
 78. The methodof claim 76, wherein application of the force in step a) is accomplishedusing a far end force motor, wherein the far end force motor applies aforce to the far end support block.
 79. An improved apparatus forforming sheet glass, wherein the apparatus includes a trough forreceiving molten glass that has sides attached to a wedged shaped sheetforming structure that has downwardly sloping sides converging at thebottom of the wedge such that a glass sheet is formed when molten glassflows over the sides of the trough, down the downwardly sloping sides ofthe wedged shaped sheet forming structure and meets at the bottom ofwedge, and wherein the improvement comprises: a) at least one inlet endstructure located at an inlet end of the trough, wherein the inlet endstructure supports a weight of the trough; and b) at least one far endstructure located at an opposite end of the trough as the inlet endstructure, wherein the far end structure supports the weight of thetrough; wherein the inlet end structure and the far end structure eachinclude at least one compression block shaped to distribute force into abottom of the trough, inducing a moment to counteract the effect of theweight of the trough such that an applied force maintains all of therefractory that comprises the trough under substantially equalcompression stress in a longitudinal direction throughout a life of aproduction campaign independent of localized thermal creep deformationof the trough at each end; such that any deformation of the formingtrough that results from thermal creep has a minimal effect on athickness variation of the glass sheet.
 80. The apparatus of claim 79,wherein the far end structure comprises: a) at least one free-floatingfar end compression block, wherein an far end compression force isapplied to a bottom of the trough by the free-floating far endcompression block; and b) at least one adjustable far end force motor,wherein the far end force motor generates the far end compression force.81. The apparatus of claim 80, wherein the far end force motor isselected from the group consisting of an adjustable spring; an aircylinder; a hydraulic cylinder; an electric motor; and a weight andlever system.
 82. The apparatus of claim 80, further comprising a cageconnected to the inlet end structure, wherein the cage applies a forceto the inlet end structure and mounts to the far end force motor toapply an equal and opposite force to the far end structure.
 83. Theapparatus of claim 82, wherein the cage comprises a low friction supportsuch that the cage is free to move in a horizontal direction.
 84. Theapparatus of claim 80, wherein the inlet end structure comprises: a) atleast one free-floating inlet compression block, wherein an inlet endcompression force is applied to a bottom of the trough by thefree-floating inlet compression block; and b) at least one adjustableinlet end force motor, wherein the inlet end force motor generates theinlet end compression force.
 85. The apparatus of claim 84, wherein theinlet end force motor is selected from the group consisting of anadjustable spring; an air cylinder; a hydraulic cylinder; an electricmotor; and a weight and lever system.
 86. The apparatus of claim 84,further comprising at least one key between each of the free-floatingcompression blocks and the trough, wherein the keys insure correctalignment of the free-floating compression blocks to the trough.
 87. Theapparatus of claim 84, wherein the free-floating far end compressionblock is set at an angle to horizontal.
 88. The apparatus of claim 84,wherein the free-floating inlet end compression block is set at an angleto horizontal.
 89. The apparatus of claim 84, further comprising a topend force motor, wherein the top end force motor is located at a top endof the far end of the trough.
 90. The apparatus of claim 89, wherein thetop end force motor is selected from the group consisting of anadjustable spring; an air cylinder; a hydraulic cylinder; an electricmotor; an adjustment screw; and a weight and lever system.
 91. Theapparatus of claim 84, further comprising a chamfer located between abottom of the wedge and a bottom edge of the free-floating inletcompression block.
 92. The apparatus of claim 84, further comprising achamfer located between a bottom of the wedge and a bottom edge of thefree-floating far end compression block.
 93. The apparatus of claim 79,wherein at least one force on the inlet end of the trough is not equalto at least one force on the opposite end of the trough as the inlet endstructure.
 94. The apparatus of claim 79, further comprising a forcemotor located at the top end of the far end, wherein the force motorgenerates a constant sealing force for a glass seal between the inflowpipe and the trough.
 95. The apparatus of claim 94, wherein the top endforce motor is selected from the group consisting of an adjustablespring; an air cylinder; a hydraulic cylinder; an electric motor; anadjustment screw; and a weight and lever system.
 96. The apparatus ofclaim 79, further comprising an inlet end adjustment screw, wherein theinlet end adjustment screw restrains the inlet end structure in alongitudinal direction.
 97. The apparatus of claim 79, furthercomprising a force motor located on the inflow pipe, wherein the forcemotor generates a constant sealing force for a glass seal between theinflow pipe and the trough.
 98. The apparatus of claim 97, wherein thetop end force motor is selected from the group consisting of anadjustable spring; an air cylinder; a hydraulic cylinder; an electricmotor; an adjustment screw; and a weight and lever system.
 99. A methodfor manufacturing glass sheets using an apparatus that includes a troughfor receiving molten glass that has sides attached to a wedged shapedsheet forming structure that has downwardly sloping sides converging atthe bottom of the wedge such that a glass sheet is formed when moltenglass flows over the sides of the trough, down the downwardly slopingsides of the wedged shaped sheet forming structure and meets at thebottom of wedge, wherein the method comprises the step of: maintaining asubstantially equal and opposite compression force to each end of abottom of the trough, independent of localized thermal creep where theforce is applied, such that any deformation of the forming trough thatresults from thermal creep has a minimal effect on a thickness variationof the glass sheet.
 100. The method of claim 99, wherein the compressionforces are independent of each other at each end. 101-113. (canceled)114. The apparatus of claim 79, wherein the far end structure comprises:a) at least one far end compression block which also supports the weightof the trough, wherein a far end compression force is applied to thebottom of the trough by the far end compression block; and b) at leastone adjustable far end force motor, wherein the far end force motorgenerates the far end compression force.
 115. The apparatus of claim114, wherein the far end force motor is selected from the groupconsisting of an adjustable spring; an air cylinder; a hydrauliccylinder; an electric motor; and a weight and lever system.
 116. Theapparatus of claim 114, further comprising a cage connected to the inletend structure, wherein the cage applies a force to the inlet endstructure and mounts to the far end force motor to apply an equal andopposite force to the far end structure.
 117. The apparatus of claim116, wherein the cage comprises a low friction support such that thecage is free to move in a horizontal direction.
 118. The apparatus ofclaim 114, wherein the inlet end structure comprises: a) at least oneinlet compression block which also supports the weight of the trough,wherein an inlet end compression force is applied to a bottom of thetrough by the inlet compression block; and b) at least one adjustableinlet end force motor, wherein the inlet end force motor generates theinlet end compression force.
 119. The apparatus of claim 118, whereinthe inlet end force motor is selected from the group consisting of anadjustable spring; an air cylinder; a hydraulic cylinder; an electricmotor; and a weight and lever system.
 120. An improved apparatus forforming sheet glass, wherein the apparatus includes a trough forreceiving molten glass that has sides attached to a wedged shaped sheetforming structure that has downwardly sloping sides converging at thebottom of the wedge such that a glass sheet is formed when molten glassflows over the sides of the trough, down the downwardly sloping sides ofthe wedged shaped sheet forming structure and meets at the bottom ofwedge, and wherein the improvement comprises: a) at least one inlet endsupport structure located at an inlet end of the trough which supportsthe trough, wherein the inlet end support structure includes acompression block which is positioned at a bottom end of the trough; b)at least one far end support structure located at an opposite end of thetrough as the inlet end support structure which supports the trough,wherein the far end support structure includes a compression block whichis positioned at the bottom end of the trough; c) an inlet endadjustment screw, wherein the inlet end adjustment screw restrains theinlet end compression block in a longitudinal direction; and d) a farend force motor, wherein the far end force motor applies a force to thefar end compression block such that a bottom of the far end of thetrough is deformed, by thermal creep, in a longitudinal direction;wherein the inlet end adjustment screw is periodically adjusted tomaintain an applied force to the inlet end compression block such that abottom of the inlet end of the trough is deformed, by thermal creep, inthe opposite longitudinal direction to the deformation at the bottom ofthe far end of the trough; such that any deformation of the formingtrough that results from thermal creep has a minimal effect on athickness variation of the glass sheet.
 121. A method for manufacturingglass sheets using an apparatus that includes a trough for receivingmolten glass that has sides attached to a wedged shaped sheet formingstructure that has downwardly sloping sides converging at the bottom ofthe wedge, at least one inlet end support structure located at an inletend of the trough which supports the trough, wherein the inlet endsupport structure includes a compression block which is positioned at abottom end of the trough, at least one far end support structure locatedat an opposite end of the trough as the inlet end support structurewhich supports the trough, wherein the far end support structureincludes a compression block which is positioned at the bottom end ofthe trough, such that a glass sheet is formed when molten glass flowsover the sides of the trough, down the downwardly sloping sides of thewedged shaped sheet forming structure and meets at the bottom of wedge,wherein the method comprises the step of: a) applying a force to the farend compression block to deform, by thermal creep, a bottom of a far endof the trough in a longitudinal direction; and b) applying a force tothe inlet end compression block to deform, by thermal creep, a bottom ofthe inlet end of the trough in an opposite longitudinal direction to thedeformation of the far end; such that any deformation of the formingtrough that results from thermal creep has a minimal effect on athickness variation of the glass sheet.
 122. The method of claim 121,wherein step b) includes the substep of periodically adjusting an inletend adjustment screw to compensate for localized thermal creep in orderto maintain a substantially constant force to the inlet end compressionblock.
 123. The method of claim 121, wherein application of the force instep a) is accomplished using a far end force motor, wherein the far endforce motor applies a substantially constant force to the far endcompression block.
 124. The method of claim 121, further comprising thestep of maintaining the forces in steps a) and b) during a life of aproduction campaign.
 125. The method of claim 76, further comprising thestep of maintaining the forces in steps a) and b) during a life of aproduction campaign.
 126. The apparatus of claim 75, wherein the far endforce motor is selected from the group consisting of an adjustablespring; an air cylinder; a hydraulic cylinder; an electric motor; and aweight and lever system.
 127. The method of claim 78, wherein the farend force motor is selected from the group consisting of an adjustablespring; an air cylinder; a hydraulic cylinder; an electric motor; and aweight and lever system.